update readme

Author: KiraPC

Diff for: README.md

@@ -15,4 +15,5 @@ fastapi-openapi-gen main:app \
     --app-dir= # The path to mounth in the python path to import the module. \
     --output-dir= # The path where store the openapi template. \
     --file-name= # The name the openapi file will assume.
+    --mock-class= # Use it to mock a package that is not usefully for the openapi generation and block the fastapi start
 ```