documentation: add CONTRIBUTING

2 jobs for documentation in 57 minutes and 39 seconds (queued for 19 seconds)
Name Stage Failure
failed
black Test There is an unknown failure, please try again
Using Docker executor with image python:3.6 ...
Using docker image sha256:0ed6db5d278e1221b2ac4565c3c8d139bb31d93177483aceb90eb200fe0e5054 for predefined container...
ERROR: Preparation failed: Error response from daemon: Conflict. The container name "/runner-e1f3acc0-project-25-concurrent-1-predefined" is already in use by container "6c299cec95676a30dffe7cd39e62033618edf5c82ad8e10c1b3a38be8ccbbd26". You have to remove (or rename) that container to be able to reuse that name.
Will be retried in 3s ...
Using Docker executor with image python:3.6 ...
Using docker image sha256:0ed6db5d278e1221b2ac4565c3c8d139bb31d93177483aceb90eb200fe0e5054 for predefined container...
ERROR: Preparation failed: Error response from daemon: Conflict. The container name "/runner-e1f3acc0-project-25-concurrent-1-predefined" is already in use by container "6c299cec95676a30dffe7cd39e62033618edf5c82ad8e10c1b3a38be8ccbbd26". You have to remove (or rename) that container to be able to reuse that name.
Will be retried in 3s ...
ERROR: Job failed (system failure): Error response from daemon: Conflict. The container name "/runner-e1f3acc0-project-25-concurrent-1-predefined" is already in use by container "6c299cec95676a30dffe7cd39e62033618edf5c82ad8e10c1b3a38be8ccbbd26". You have to remove (or rename) that container to be able to reuse that name.