Docs: add description of supported functionality

[loj]

Includes links to the datalad docs for the supported commands.

Towards #45

[adswa]

I would propose to merge this quickly, so that it doesn't breed conflicts. Maybe a future update could mention that this command set is what the "simple" view supports (I think the complete view has the entire command suite, right?)

[jsheunis]

Adina said what I wanted to say. same 👍