Add contribution guide

CONTRIBUTING.md

+
+- master - Development branch
+- gajim_1.0 - Stable Gajim 1.0.x branch
+- gajim_0.16 - Stable Gajim 0.16.x branch
+
+If you are not familiar with Git please read the [HowTo](https://dev.gajim.org/gajim/gajim/wikis/howtogit)
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