started contribution guide

CONTRIBUTING.md

+# Contribution guidelines
+
+#### Always work on feature branches
+
+Please branch from `develop` to create a new _feature_ branch.
+
+Please create a new _feature_ branch for every new feature or fix.
+
+#### Do not commit directly to `master` or `develop`.
+
+Use your _feature_ branch.
+
+Please rebase your work against the `develop` before submitting a merge reqeuest.
+
+#### Make the CI happy :-)
+
+Only branches which pass the CI can be merged.
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