doc for devs

HACKING.md

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+The instructions in README.md are fine when you only want to run. However, when developing, you might need to
+know some more tricks.
+
+The goal of our build manager (`dodo.py`) is to make a single call to `doit`, without arguments, always a sane
+and correct choice. However that could not always be the case.
+
+Let's organize our explanation into "iteration loops": you are typically developing on some aspect of it, and
+not on the entire setup. Based on this, you'll need to do different things.
+
+## Loops
+
+### Hacking code on tt-rss or rss-bridge
+
+If you are changing PHP code, well, this is the easiest part.
+Run `doit`, and you should have everything running, and logs showing up.
+Then, with your favourite editor, edit the PHP files you prefer. You can then refresh what you want in the
+browser, and everything should be automatically changing. No other action required!
+
+### Changing nginx configuration
+
+Currently, there is no way to reload nginx, even though it should not be impossible, theoretically, to do it.
+So edit `docker/nginx.conf` with your favourite editor then run `doit`. On the next change, give `ctrl-c` to
+`doit` and then again run `doit`.
+
+This workflow could be improved, being currently a bit too slow for such an easy thing.
+
+### Change some docker file
+
+When you change the dockerfile you need to build everything again. Doit should notice this, and just running
+`doit` should build everything again. If for some reason this is not the case, `doit clean build` will clean
+all of our artifacts; so that running `doit` will surely rebuild everything.
+
+Notice that building is a bit slow. There is no way to improve this, so just keep it in mind!