Added dev instructions about our Git/issue tracking

Author: nicolas-raoul

README.md

@@ -83,6 +83,22 @@ Captured files are not currently stored within the app, but are passed by conten
 
 Thumbnail images are not currently cached.
 
+## Volunteers welcome! ## 
+
+We are always looking for volunteers, feel free to step in! It is very easy:
+
+1. Fork the repository and clone it to your computer, then follow the build instructions above.
+2. Choose an [unassigned issue](https://github.com/commons-app/apps-android-commons/issues?q=is%3Aopen+is%3Aissue+no%3Aassignee) that sounds interesting to you.
+3. Read the issue's comments to make sure you understand what is the bug, or what feature is being proposed.
+4. Write a "I start working on this" comment on the issue
+5. Write the code :-)
+6. Commit and push
+7. Go to your fork's Github webpage, select the "Pull Requests" tab and click "create a pull request", as a comment, write something like "Fix for issue #12345 crash when rotating screen", then submit the pull request.
+8. Within a few hours or days, a core developer will review your patch, and either merge it or suggest a few corrections.
+9. If you change your mind, or if it is too difficult, no problem, just write "Sorry I don't work on this anymore" on the issue, if possible including feedback (for instance what approaches failed) and ideas.
+
+Thanks a lot!
+
 ## Translating the app ## 
 
 Thanks to the translation work of many volunteers this app is available in a multitude of languages.