docs: add CONTRIBUTING.md info for leonardo update

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# contributing to terra-azure-relay-listeners
+
+## Updating leonardo
+
+In order to finish releasing your changes, you will need to update `leonardo`
+with your commit [git-sha from `main`](https://github.com/DataBiosphere/terra-azure-relay-listeners/commits/main/).
+
+At the time of this writing, there are two files that need to be modified with
+the git-sha corresponding to your commit on `main`.
+
+- [reference.conf](https://github.com/databiosphere/leonardo/blob/develop/http/src/main/resources/reference.conf#L219)
+- [ConfigReaderSpec.scala](https://github.com/databiosphere/leonardo/blob/develop/http/src/test/scala/org/broadinstitute/dsde/workbench/leonardo/http/ConfigReaderSpec.scala#L71)
+
+To ensure you have covered all references, feel free to execute the following
+from `leonardo/`'s root directory:
+
+```shell
+git grep terra-azure-relay-listeners
+```
+