Document that `wp_before_creating_autosave` is be to added in Trac Ticket 20299

[grappler]

It may be better to only add wp_before_creating_autosave once core-#20299 is closed and the filter is in core.

https://github.com/adamsilverstein/wp-post-meta-revisions/blob/ca459926808b025c256654962e577c91597dd0e0/wp-post-meta-revisions.php#L36