add docker hub badge

Author: p1c2u

README.rst

@@ -2,6 +2,8 @@
 OpenAPI Spec validator
 **********************
 
+.. image:: https://img.shields.io/docker/v/p1c2u/openapi-spec-validator.svg?color=%23086DD7&label=docker%20hub&sort=semver
+     :target: https://hub.docker.com/r/p1c2u/openapi-spec-validator
 .. image:: https://img.shields.io/pypi/v/openapi-spec-validator.svg
      :target: https://pypi.python.org/pypi/openapi-spec-validator
 .. image:: https://travis-ci.org/python-openapi/openapi-spec-validator.svg?branch=master