Create CONTRIBUTING.md

First draft of contribution guidelines

CONTRIBUTING.md

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+This document speciffy hints and good practices for source code contributions.
+
+AndroidAPS is community effort and all contributions are welcome! If you wish help us improving AndroidAPS - please read and try to adhere to 
+this guidelines, to make the development and process of change aproval as smooth as possible :) 
+
+General rules
+=============
+
+* There are plenty of ways you can help, some of them are listed on wiki: 
+  https://androidaps.readthedocs.io/en/latest/EN/Getting-Started/How-can-I-help.html
+* If you wish to help with documentation or translating: 
+  https://androidaps.readthedocs.io/en/latest/EN/translations.html
+  
+Development guidelines
+======================
+
+Commiting Changes / Pull Requests
+---------------------------------
+
+1. Make fork of repository on github
+2. Create separate branch for each feature, branch from most recent dev
+3. Commit all changes to your fork
+4. When ready, rebase on top of dev and make pull request to main repo
+
+Naming Conventions for Pull Requests / Branches
+-----------------------------------------------
+
+TODO
+
+Translations
+------------
+
+* If possible, always use Android translation mechanism (with strings.xml and @strings/id) instead of hardcoded texts
+* Provide only English strings - all other languages will be crowd translated via Crowdn https://translations.androidaps.org/
+
+Hints
+-----
+
+* Start small, it is easier to review smaller changes that affect fewer parts of code 
+* Take a look into Issues list (https://github.com/MilosKozak/AndroidAPS/issues) - maybe there is somthing you can fix or implement
+* For new features, make sure there is Issue to track progress and have on-topic discussion
+* Reach out to community, discuss idea on Gitter (https://gitter.im/MilosKozak/AndroidAPS)
+* Speak with other developers to minimise merge conflicts. Find out who worked, working or plan to work on speciffic issue or part of app