docs: add contributing guidelines

CONTRIBUTING.md

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+# Contributing to speedbump
+
+## Bugs and feature requests (Issues)
+
+1. If you have found a bug, please check existing Issues prior to opening a new one.
+2. When reporting a bug via issues, please provide commit # (or output of `speedbump --version` if you are using a pre-built binary form a GitHub release).
+3. A bug report should include accurate description of the steps required to reproduce it.
+4. Feature requests are welcome!
+
+## Pull requests
+
+### Commit naming
+
+This project uses [conventional commits](https://www.conventionalcommits.org/).
+
+### WIP changes
+
+If you encounter a problem while having work-in-progress changes in your repo, don't hesitate to open a draft pull request.
+
+Thanks!