Add README.md explaining repo merge.

Author: mattclay

README.md

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+**NOTE:** As of Ansible 2.3, modules are now in the
+[main Ansible repository](https://github.com/ansible/ansible/tree/devel/lib/ansible/modules).
+
+See the [repo merge guide](https://docs.ansible.com/ansible/dev_guide/repomerge.html) for more information.
+
+This repo still exists to allow bug fixes for `stable-2.2` and older releases.