Add CONTRIBUTING.md

Author: nhooyr

.github/CONTRIBUTING.md

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+# Contributing Guidelines
+
+Please split up changes into several small descriptive commits.
+
+Please capitalize the first word in the commit message and ensure it is
+descriptive.
+
+The commit message should use the verb tense + phrase that completes the blank in
+
+> This change modifies websocket to ___________
+
+Be sure to link to an existing issue if one exists. In general, try creating an issue
+before making a PR to get some discussion going and to make sure you do not spend time
+on a PR that may be rejected.
+
+Run `test.sh` to test your changes. You only need docker and bash to run tests.