add regenerate instructions

Author: gijzelaerr

README.md

@@ -7,3 +7,10 @@ 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
 ```
+
+to regenerate install `schema_salad` package and run:
+
+```
+schema-salad-tool --codegen python \
+    https://raw.githubusercontent.com/common-workflow-language/common-workflow-language/master/v1.0/CommonWorkflowLanguage.yml
+```