Add contribution guide

CONTRIBUTING.md

+### Branches
+
+- master (Python2/Gtk2 for Gajim 0.16.x)
+- gtk3   (Python3/Gtk3 for Gajim master branch
+
+### Git Commit Messages
+
+* Start your commit message with the Plugin name, example: [omemo] Added ..
+* Use the present tense ("Add feature" not "Added feature")
+* Limit the first line to 50 characters or less, add whatever you feel necessary
+in the extended commit message