Updated README: auth problems solution

README.md

@@ -35,6 +35,16 @@ permissions](https://gitlab.cern.ch/help/permissions/permissions.md): users in
 **alice-off** have **Developer** permissions from the table.
 
 
+### First login
+
+You must login at least once on the GitLab web interface before being able to
+push from the command line. In case push fails with an "authentication
+problem", it means you need to go once again on GitLab web and login from there.
+
+This is because egroups are synchronized with GitLab groups only when logging in
+from the web interface.
+
+
 GitLab at CERN
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