Contribution guidelines

Author: jdee

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# Guidelines for contributors
+
+Minimally, all contributions must pass CI testing. To test yourself locally:
+
+```bash
+bundle exec rake
+```
+
+Note that many RuboCop issues can be automatically corrected. If your
+contribution fails RuboCop, try:
+
+```bash
+bundle exec rubocop -a
+```