Add contribution guide

CONTRIBUTING.md

+Contributions
+-------------
+
+If you make a change in the toolbox, that others can benefit from please make a merge request.
+
+If you can, please submit a merge request with the fix or improvements including tests.
+
+The workflow to make a merge request is as follows:
+
+
+Create a feature branch, branch away from master
+Write tests and code
+Push the commit(s) to your fork
+Submit a merge request (MR) to the master branch of
+Link any relevant issues in the merge request description and leave a comment on them with a link back to the MR
+Your tests should run as fast as possible, and if it uses test files, these files should be as small as possible.
+Please keep the change in a single MR as small as possible. Split the functionality if you can
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