Update/docs

[jeffpaul]

This PR:

• adds the Update URI header value to avoid naming conflicts should another plugin get published to WP.org before this plugin does (if this plugin gets published there, the Update URI header can be removed)
• removes the Requires PHP and Requires at least header values from readme.txt as they're no longer needed there as of WP 5.8
• adds the Requires at least header value to gatherpress.php as the correct location for that
• reduces the length of the short description in readme.txt to be under 150 characters per requirements on WP.org
• moves the former short description to the long description in readme.txt
• tweaks the copy and links in the contribution sections in README.md and readme.txt
• adds the Screenshots and Changelog sections to readme.txt (with a todo reference to add in screenshots as the come available from the work in Add and complete readme.txt #406).

Relates to #50, #406.