Add dependencies on signon readme

signon/README.md

@@ -24,6 +24,12 @@ Represent authentication providers and how to interact with them.
 
 # Using signon
 
+## Dependencies
+
+```
+apt install python3-jwcrypto python3-requests-oauthlib python3-gitlab
+```
+
 ## Adapt the User model
 
 Provide `get_user_model().objects.get_housekeeper()` to return a user object to