Document conventions for command line args for tox core and plugins

[obestwalter]

As @hpk42 mentioned certain conventions have been growing to prevent command line args clashes between tox core and plugins, which boil down to

• tox -> lowercase switches and long options
• plugins -> only uppercase switches (and no long options? Not sure about that yet)

This should be documented in the plugins docs.

[gaborbernat]

I'll take this.

[gaborbernat]

I don't think this will solve things reliably. I would suggest instead plugin option should start with the plugin part, e.g. tox-travis should have --travis--a, tox-docker should have --docker-b and so on.

[gaborbernat]

We can't really enforce any of this however we can make recommendations :+1: