Add contributing file

contributing.md

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+# Contributing to Jepsen
+
+Hi there, and thanks for helping make Jepsen better! I've got just one request:
+start your commit messages with the *part* of Jepsen you're changing. For
+instance, if I made a change to the MongoDB causal consistency tests:
+
+> MongoDB causal: fix a bug when analyzing zero-length histories
+
+Namespaces are cool too!
+
+> jepsen.os.debian: fix libzip package name for debian stretch
+
+If you're making a chance to the core Jepsen library, as opposed to a specific
+database test, you can be more concise:
+
+> add test for single nemesis events
+
+Jepsen's a big project with lots of moving parts, and it can be confusing to
+read the commit logs. Giving a bit of context makes my life a lot easier.
+
+Thanks!