Add CONTRIBUTING guide

CONTRIBUTING.md

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+#### We love pull requests. Here's a quick guide
+
+1. If you are a member of thessaloniki/thessrb-members team you can skip
+   2.
+2. Fork the repository (want to be a member and not have to fork? [Check
+   how]())
+3. Create a topic branch, do some work, push it, and create a pull
+   request.
+
+#### Things we like
+
+* Nice title and a complete description of what and why in the pull
+  request body.
+* Check what is already in the repository to see how you should format
+  things.
+* If you are contributing written text, we like the Markdown format.
+
+#### Things to avoid
+
+* Spaces or underscores in filenames (we prefer dashes to separate
+  words, but single word filenames are better if possible)
+* Don't commit large binary files and if you have to commit smaller
+  ones, make sure they are not going to be updated often. For all other
+cases, it's better to host the file somewhere else and provide a link to
+it (for example presentations etc.)
+