Create README.md

Author: mr-c

README.md

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+Requires Python 3.6.x or Python 3.7
+
+``` bash
+git clone https://github.com/common-workflow-language/cwl-utils.git
+virtualenv -p python3.6 venv3.6
+source venv3.6/bin/activate
+pip install cwl-utils/docker_cache_pull/requirements.txt
+python docker_cache_pull/docker-extract.py path_to_my_workflow.cwl
+```