Usage update for README.md

README.md

 # Canvas API Postman
 
-Script to extract the current API documentation from Canvas and generate an importable Postman collection.
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+Script to extract the current API documentation from Canvas and generate an importable Postman collection.
+
+`python3 canvas-api-postman.py`
+
+Outputs an 'output.json' file that is used for import into Postman
+
+In Postman choose Import -> Import File -> navigate to this folder
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