doc: explain the GITLAB_TOKEN

Closes #39

README.rst

@@ -20,6 +20,16 @@ Example:
 
     GITLAB_TOKEN=... python3 doctor_hg.py fix-README
 
+This GITLAB_TOKEN is an heptapod/gitlab access token. You can manage yours here
+https://forge.extranet.logilab.fr/-/profile/personal_access_tokens
+
+Using your token will open MR with your user. If you have enough access this is
+the prefered way to do that so we can have an idea on who and how those MR have
+been created in case we need to do some debugging.
+
+There is also a cube doctor account with it's own access token, you can ask
+people in the team to have it if needed.
+
 Explanation
 -----------