add contribution guide

Author: karlosss

CONTRIBUTING.md

@@ -0,0 +1,15 @@
+# Contributing
+
+All contributions are done in form of pull requests - fork the repository, do the work, and then
+create a pull request.
+
+In case of implementing a new feature, make sure to add testcases for it.
+
+The tests are run automatically for each pull request using a CI/CD pipeline. To run the test
+locally, you can use `run_tests.sh`. Refer to the contents of the script for more information.
+
+When making a pull request, make sure that the commits on the branch all make sense and the
+history of the branch is (at least somewhat) clean. If this is not the case, 
+`git rebase --interactive` is your friend.
+
+Thank you very much for your contributions!