Add contributing.md // FREEBIE

This file is recognized by github and presented to users who are trying
to open issues and submit pull requests.

Hopefully this more prominent placement will help deter people from
opening issues/prs for translation fixes.

contributing.md

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+##Translations
+
+Please do not submit issues or pull requests for translation fixes. Anyone can update the translations in [Transifex](https://www.transifex.com/projects/p/textsecure-official/). Please submit your corrections there.