Add more details on how the options of the submit directive affects the configurations in A+.

[jaguarfi]

The Rubyric module covers many topics on how to set up Rubyric, but a section on how the options added to the submit directive affect the configurations in A+, e.g. _external, min_group_size, max_group_size, needs to be added.
This issue is created after the discussion of the PR #25. However, the main concern was related to the manual configuration required in Rubyric when the min_group_size, max_group_size were added. This issue is also linked to apluslms/a-plus-rst-tools#70