Add contributing doc.

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+# Contributing to Erlex
+
+Thank you for considering contributing to Erlex! No matter how big or
+small, every contribution helps make this a better project. We
+absolutely appreciate any and all help.
+
+## Issues
+
+Please file issues [here](https://github.com/asummers/erlex/issues)
+
+## Documentation
+
+Documentation is the gift that keeps on giving. If you notice
+something even as small as a typo or an ambiguous sentence, please
+file an issue or submit a pull request!
+
+## Development Contributions
+
+Take a look at the open issues
+[here](https://github.com/asummers/erlex/issues) and post that you
+plan on working on an issue, so we can discuss approaches and make
+sure there isn't duplicated work happening.
+
+Before submitting a pull request, make sure all of the following pass:
+
+```
+mix deps.get
+mix format --check-formatted
+mix compile --force --warnings-as-errors
+mix credo list --strict
+mix test
+```
+
+Steps:
+1) Fork it!
+2) Create a new branch off of master in your fork and do your work
+3) Squash your commits to a single commit. If you don't know how to do
+this, that's okay. We will work through it in pull request.
+4) Make sure any documentation has been updated and all checks are passing.
+5) Submit a pull request. Thank you! :heart:
+
+## Code of Conduct
+
+Be sure to read and follow the [code of conduct](https://github.com/asummers/erlex/blob/master/code-of-conduct.md).

README.md

@@ -60,3 +60,10 @@ iex> Erlex.pretty_print_contract(str)
   binary()
 ) :: Plug.Conn.t()
 ```
+## Contributing
+
+We welcome contributions of all kinds! To get started, click [here](https://github.com/asummers/erlex/blob/master/CONTRIBUTING.md)
+
+## Code of Conduct
+
+Be sure to read and follow the [code of conduct](https://github.com/asummers/erlex/blob/master/code-of-conduct.md).