Add readme stub

Author: yosifkit

README.md

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+# About this Repo
+
+This is the Git repo of the official Docker image for [postgres](https://registry.hub.docker.com/_/postgres/). See the
+Hub page for the full readme on how to use the Docker image and for information
+regarding contributing and issues.
+
+The full readme is generated over in [docker-library/docs](https://github.com/docker-library/docs),
+specificially in [docker-library/docs/postgres](https://github.com/docker-library/docs/tree/master/postgres).