doc: explain the GITLAB_TOKEN

Closes #39
1 job for branch/default in 2 minutes and 40 seconds (queued for 5 seconds)
Status Name Job ID Coverage
  Test
failed auto-mr #417681

00:02:40

 
Name Stage Failure
failed
auto-mr Test
  File "/builds/cubicweb/cube-doctor/cube_doctor/__init__.py", line 721, in workflow
for cube in cubes:
File "/builds/cubicweb/cube-doctor/cube_doctor/__init__.py", line 92, in get_all_cubes
for x in cnx.projects.list(all=True, as_list=False)
File "/usr/local/lib/python3.7/dist-packages/gitlab/exceptions.py", line 306, in wrapped_f
raise error(e.error_message, e.response_code, e.response_body) from e
gitlab.exceptions.GitlabListError: 500: 500 Internal Server Error
Cleaning up file based variables
ERROR: Job failed: command terminated with exit code 1