Add contributing guidelines (#140)

* Add contributing guidelines

CONTRIBUTING.md

@@ -0,0 +1,17 @@
+# Contributing to MC<sup>2</sup>
+
+## Reporting bugs and asking questions
+
+You can ask questions, bring up issues, or garner feedback through the following channels:
+
+1. [Slack](https://join.slack.com/t/mc2-project/shared_invite/zt-rt3kxyy8-GS4KA0A351Ysv~GKwy8NEQ)
+2. [GitHub Issues](https://github.com/mc2-project/mc2/issues)
+3. Email: send an email to [email protected]
+
+## To contribute a patch
+
+1. Break your work into small, single-purpose patches if possible. It's much
+   harder to merge in a large change with a lot of disjoint features.
+2. Submit the patch as a GitHub pull request against the master branch.
+3. Make sure that your code passes the automated tests.
+4. Make sure that your code passes the linter. Run `pip3 install pre-commit; pre-commit install` to create a git hook that will run the linter before you push your changes.