Add usage instructions to README

Signed-off-by: Stephen Finucane <[email protected]>

Author: stephenfin

README.rst

@@ -6,5 +6,15 @@ pyenv container image for use in `Patchwork`__ development and CI.
 
 Based on `vicamo/docker-pyenv`__.
 
+Usage
+----
+
+This is only intended for use with Patchwork. If you have reason to use it
+though, you can pull it like so:
+
+.. code-block:: shell
+
+    $ docker pull ghcr.io/getpatchwork/pyenv:latest
+
 .. __: https://github.com/getpatchwork/patchwork
 .. __: https://github.com/vicamo/docker-pyenv