COMPATIBILITY.md

Create a starting compatibility guide that can help maintainers support
the on-going development of this client.

Signed-off-by: Lawrence Jones <lawrjone@gmail.com>

COMPATIBILITY.md

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+# Compatibility
+
+We aim for the Prometheus Ruby client to be compatible with all supported
+versions of Ruby, across the MRI and JRuby platforms.
+
+Any Ruby version that has not received an End-of-Life notice (e.g.
+[this notice for Ruby 2.1](https://www.ruby-lang.org/en/news/2017/04/01/support-of-ruby-2-1-has-ended/))
+is supported.
+
+To ensure we're meeting these guidelines, we test the client against all
+supported versions, as specified in our [build matrix](.travis.yml).
+
+# Deprecation
+
+Whenever a version of Ruby fails out of support we will mirror that change in
+the Prometheus Ruby client by updating the build matrix and releasing a new
+major version.
+
+At that point we will close any issues that affect only the unsupported version,
+and may choose to remove any workarounds from the code that are only necessary
+for the unsupported version.
+
+The major version bump signals the break in compatibility. If the client happens
+to work on unsupported versions of Ruby this is by chance, and we wouldn't
+consider that version to be officially supported.