Add documentations for Wayback Citations w/ Playback to README

[mekarpeles]

The general idea is you can now go to pragma.archivelab.org and enter a url and a css ID.

The form will kick off a web annotation {"annotation": {"id": "..."}} which references the css ID specified in the form. A wayback snapshot will then be kicked off via pragma and result in an entry being created (having a unique wayback pragma ID) within the pragma wayback snapshots table, which is essentially a dictionary which references the wayback snapshot's url (once it's saved via Save Page Now) and a link to the annotation's ID in pragma (if an annotation was created).

Submitting through this form will then redirect the user to https://pragma.archivelab.org/playback/ :wayback pragma ID which should highlight and bring the user directly to this cited region.