Document GitLab backend usage

[xurizaemon]

There is no documentation for the GitLab data source. This is based off the GitHub docs but I've reviewed it and believe it is correct for GitLab.

Additional minor changes here:

• Includes minor inline docs changes to perceval/backends/core/gitlab.py for accuracy (IMO).
• Lowercases the gitlab argument group for consistency with other argument groups as in perceval gitlab --help, other groups are lowercase while GitLab was not.
• For the python usage example, I made it print GitLab's award emoji where the GitHub example demonstrated using returned data to show item type.
• Fix project README example usage for Gitlab referring to a non-existent project.

This PR would need updating if #801 landed first, and I will update whichever is required when one or the other lands 😁

[coveralls]

Coverage decreased (-0.07%) to 97.792% when pulling fbde5895a2c36d1b0630af725bde00c134c3dca1 on xurizaemon:docs-gitlab into e8869879ca93e0a53e663f82d0674eb2194c6a44 on chaoss:master.

[xurizaemon]

Great, will do! Thanks.

[xurizaemon]

Hi @sduenas - rebased to account for #801, removed CHANGELOG entry which has caused the check-changelog check to fail as it should 😁

[xurizaemon]

Sidebar: Is that Coveralls check accurate? I'm not clear how this PR seemed to at first increase, and now decrease, coverage. But I'm not familiar with Coveralls so my expectation that it reflects test coverage vs LoC may be incorrect.

[sduenas]

Sidebar: Is that Coveralls check accurate? I'm not clear how this PR seemed to at first increase, and now decrease, coverage. But I'm not familiar with Coveralls so my expectation that it reflects test coverage vs LoC may be incorrect.

Unless the change of coverage is big, I usually ignore it. Sometimes it doesn't calculate very well small changes or when there aren't changes at all.

[sduenas]

Merged

[sduenas]

Thanks for your contribution @xurizaemon