Add `npm run postinstall` to README (closes #92)

Appears to be required for `patch-package` to do its thing (at least patch-package suggests you use `postinstall`, not really sure why though since it isn't ran on `npm install`).

README.md

@@ -24,6 +24,7 @@ or from git:
 git clone https://github.com/johni0702/mumble-web
 cd mumble-web
 npm install
+npm run postinstall
 npm run build
 ```