Add explanation about why proof graphs are used

[msporny]

The VC Data Model added an explanation related to VC graphs in PR w3c/vc-data-model#1280. The Data Integrity specification should do the same.

[msporny]

This request is also related to a request by @iherman in this comment: w3c/vc-data-model#1403 (review)

[iherman]

This is also related to the discussion in w3c/vc-data-model#1397 (review): the fact that the proof property's value is a named graph, i.e., that the property refers to a container property in JSON-LD parlance, must be clearly specified in the DI specification (it was, so far, part of the VCDM spec, but has been removed as part of w3c/vc-data-model#1397).

[msporny]

PR #270 has been raised to address this issue. This issue will be closed once PR #270 has been merged.

[msporny]

PR #270 has been merged, closing.