Speculative Loading readme needs improvement

[westonruter]

Feature Description

See feedback from @felixarntz in #1155 (review)

A Google Doc has been shared for collaboration.

[felixarntz]

I added some references and one concrete wording suggestion to the readme in the doc. Let me know what you think!

[westonruter]

Looks like this won't land in time for Monday's release. Since it's the only substantive user-facing change in plugin's milestone, I'm not planning to do a release for this plugin on Monday. I've removed the milestone's due date from being set to Monday.

[felixarntz]

This got stalled a bit, apologies. I just took a fresh look at the readme doc and added a few more suggestions.

Let's keep using suggestions (without approving them) so that it's easy to spot the changes and paste them back into the readme.txt file later.

I'd appreciate another review from you @tunetheweb @westonruter.