docs: add CONTRIBUTING.md

Author: dbongo

CONTRIBUTING.md

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+# Contributing
+
+Bug reports and pull requests are welcome on GitHub at https://github.com/dbongo/callable-mixin.
+
+Here's how to get started:
+
+1. Fork the repo, then `git clone` your fork.  
+2. Create a feature branch: `git checkout -b feat/my-feature`.  
+3. Run tests: `bundle exec rspec`.  
+4. Make your changes, commit, and push up your branch.  
+5. Open a Pull Request describing your change.