Add contributing documentation

CONTRIBUTING.md

@@ -0,0 +1,14 @@
+# Contributing
+
+This is an open source project, and we appreciate your help!
+
+We use the GitHub issue tracker to discuss new features and non-trivial bugs.
+
+In addition to the issue tracker, [#journeys on
+Slack](https://dwopen.slack.com) is the best way to get into contact with the
+project's maintainers.
+
+To contribute code, documentation, or tests, please submit a pull request to
+the GitHub repository. Generally, we expect two maintainers to review your pull
+request before it is approved for merging. For more details, see the
+[MAINTAINERS](MAINTAINERS.md) page.