Add note to readme about Gutenberg support

[jeffpaul]

Is your enhancement related to a problem? Please describe.
Gutenberg-based exports includes some additional metadata/details that a Classic Editor-based export does not.

Describe the solution you'd like

• Update the readme to call out differences between exports from the Classic Editor and Gutenberg.
• switch from using links to Adam Silverstein's personal Gists for XML samples to ones hosted either in-repo or on a 10up controlled web entity
• test importing Classic Editor and Gutenberg exports into InDesign, capture differences, add to readme as examples

Designs
n/a

Describe alternatives you've considered
none

Additional context
Background details from 10up Slack.

[jeffpaul]

Part of this has been resolved with updates in v1.1.1, but will need some more testing/updates that will come in a future release. So I'll punt this to v1.2.0 in hopes of fully resolving this issue in that release.

[jeffpaul]

This issue still has the final item remaining to be resolved, so anyone with InDesign installed who can help test would be greatly appreciated!:

• test importing Classic Editor and Gutenberg exports into InDesign, capture differences, add to readme as examples