Translating: improve instructions to update

Add Github cheat sheet instruction for carry-on working with
translation.

Contributing-to-Mastodon/Translating-Github-Cheat-Sheet.md

@@ -1,7 +1,9 @@
 Translating GitHub Cheat Sheet
 ===========
 
-## Install Git
+## Preparation
+
+### Install Git
 1. [Install Git](https://git-scm.com/book/en/v2/Getting-Started-Installing-Git) based on what operating system you have
 2. [Add your identity](https://git-scm.com/book/en/v2/Getting-Started-First-Time-Git-Setup) to your git environment. (If you use an email associated with your GitHub account your profile will show a heat map of your commit activity).
 ```bash
@@ -10,12 +12,15 @@ git config --global user.email johndoe@example.com
 ```
 **NOTE: Use quotations around your name because of the space.**
 
-## Fork The Repository
+### Fork The Repository
 1. Navigate to [Mastodon's GitHub Page](https://github.com/tootsuite/mastodon)
 2. Click `Fork` in the top right.
 3. There will now be a copy of the repository on your account.
 
-## Clone The Repository
+
+## Start Translating
+
+### Clone The Repository
 1. Open Terminal on your computer and navigate to a place where you want to store the project using `cd` (ex. `cd ~/Documents`)
 2. Download a copy of the project using
 ```
@@ -31,7 +36,7 @@ git checkout -b locale
 ```
 5. Write your awesome translations following the [translating guide](./Translating.md#translating-1)
 
-## Push To Your Repository
+### Push To Your Repository
 1. Open Terminal and navigate to where you stored mastodon (ex. `cd ~/Documents/mastodon`)
 2. Type `git status` to see files you have modified or added.
 3. We need to add those to the commit we're about to make and we can add them all at once with
@@ -48,13 +53,13 @@ You'll want to make your commit message something short, but meaningful as it's
 git push
 ```
 
-## Pull Request
+### Pull Request
 1. [Click this](https://github.com/tootsuite/mastodon/compare) to start a new pull request, or navigate to [Mastodon's GitHub Page](https://github.com/tootsuite/mastodon), click `Pull requests`, and `New pull request`
 2. On the Compare Changes page you will want to click `compare across forks`
 3. The base fork should be `tootsuite/mastodon:master` and the head fork should be `YOURUSERNAME/mastodon:YOURBRANCH`. This will show you a summary of changed files.
 4. Click `Create pull request` and explain your changes.
 
-## Finished
+### Finished
 Congratulations! Your pull request will be reviewed for merging into the repository.
 
 If you had any questions or confusions during this process please submit a [documentation issue](https://github.com/tootsuite/documentation/issues) or [pull request](https://github.com/tootsuite/documentation/pulls) clarifying confusing areas.
@@ -62,3 +67,141 @@ If you had any questions or confusions during this process please submit a [docu
 If things get overwhelming, copy out your translation work, delete the project locally, remove the fork from your account, and start the process over again.
 
 If you would prefer to not work in a Terminal, there are [GUI clients](https://git-scm.com/downloads/guis) available for git.
+
+
+## Carry on Translations
+
+OK. You have finished your translation long time ago. And Mastodon improved
+over time. Now you want to update the translation.
+
+There have been some time before your last work. Although you have a copy
+of the source code, but it is just an outdated version.
+
+In you idle time,
+* some other people could have added translations to your language;
+* structure of the translation file could have been changed.
+
+You'll need to clean up your workspace to work. Naturally you need to:
+
+1. [Get the latest source code](#get-the-latest-source-code)
+2. [Branch out again](#branch-out-again)
+3. [Work on your translation](#work-on-your-translation)
+4. [Push to your repository](#push-to-your-repository-1)
+5. [Pull Request](#pull-request-1)
+
+### Get the Latest Source Code
+
+#### 1. Make sure you have the `upstream` bookmark right.
+
+If you're not sure, run this:
+
+```
+git remote add upstream https://github.com/tootsuite/mastodon.git
+```
+
+If you get the following message, ignore it:
+```
+fatal: remote upstream already exists.
+```
+
+To verify, you can run this:
+```
+git remote get-url upstream
+```
+
+The output should be:
+```
+https://github.com/tootsuite/mastodon.git
+```
+
+#### 2. Fetch source code from `upstream`
+
+```
+git fetch upstream master
+```
+
+The output should be:
+```
+From https://github.com/tootsuite/mastodon
+ * branch            master     -> FETCH_HEAD
+```
+
+#### 3. Sychronize your `master` branch with Mastodon's `master` branch
+
+```
+git reset --hard upstream/master
+```
+
+The output should be something similar to this:
+```
+HEAD is now at abcd123 Some commit message
+```
+
+### Branch Out Again
+
+#### 1. Decide the branch name
+
+It is recommended to use RFC5646 language tag as your translation
+branch name. If not sure, use `locale` as the branch name.
+
+#### 2. Remove existing branch, if any.
+
+```
+git branch -D BRANCHNAME
+```
+
+If you get a message similar to either of the below ones, you're fine:
+```
+error: branch 'BRANCHNAME' not found.
+```
+or
+```
+Deleted branch BRANCHNAME (was abcd123).
+```
+
+#### 3. Branch out from master
+
+```
+git checkout master
+git checkout -b BRANCHNAME
+```
+
+### Work on your translation
+
+Write your awesome translations following the [translating guide](./Translating.md#translating-1)
+
+### Push to your repository
+
+#### 1. Review the Changes to Push
+
+Open Terminal and navigate to where you stored mastodon (ex. `cd ~/Documents/mastodon`)
+
+Type `git status` to see files you have modified or added.
+
+We need to add those to the commit we're about to make and we can add them all at once with
+
+```bash
+git add .
+```
+
+#### 2. Create a Commit of the Changes
+
+We can commit everything to our local copy with
+```
+git commit -m "YOUR MESSAGE HERE"
+```
+You'll want to make your commit message something short, but meaningful as it's visible to everyone. A good example would be, "Added French localization".
+
+#### 3. Upload the Changes in the Commit
+
+Now you can push your changes to your remote copy of mastodon using
+```
+git push
+```
+
+### Pull Request
+
+1. [Click this](https://github.com/tootsuite/mastodon/compare) to start a new pull request, or navigate to [Mastodon's GitHub Page](https://github.com/tootsuite/mastodon), click `Pull requests`, and `New pull request`
+2. On the Compare Changes page you will want to click `compare across forks`
+3. The base fork should be `tootsuite/mastodon:master` and the head fork should be `YOURUSERNAME/mastodon:YOURBRANCH`. This will show you a summary of changed files.
+4. Click `Create pull request` and explain your changes.