Document use of Archivematica's post-store AIP callback in integration

[bonniegee]

Is your feature request related to a problem? Please describe.

V1.1 uses Archivematica's storage service callbacks, which can be configured to deliver POST requests on various events, to identify when an AIP or DIP has been stored (see #43, #144).

Describe the solution you'd like

To facilitate deployment, a description of the integration--including where things may go wrong, and if any special configs need to be provided--should be documented in Gemini, likely in the ReadMe.

Additional context

See this Archivematica issue.