Update screenshots section of readme.txt

New screenshots are being directly added to the plugins SVN inside the `assets` folder.
We need to update the readme.txt to show all of the new screenshots on WordPress.org.

See #5568

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8962 cdf35c40-ae34-48e0-9cc9-0c9da1808c22