Add simple contributing guide

Author: cptartur

CONTRIBUTING.md

@@ -0,0 +1,8 @@
+# Building and Contributing Guide
+
+To contribute to this project, feel free to open a PR introducing your changes.
+
+Make sure that your PR is properly formatted. It can be automatically done with `npm run fmt`.
+
+Make sure that you've run `npm run build` command and committed any changes introduced in `dist` directory.
+This step is necessary for your changes to actually be used in the action.