README: More practical usage examples

[pesse]

While all necessary information is available, it might help folks new to cli to have a comprehensive list of usage examples

• Using suite-path instead of package
• call tests of different users
• call certain tags
• call specific contexts

@jgebal Do you think it might be valuable to add a notice that cli offers the full functionality of ut.run-API (besides a_force_manual_rollback)?