Update readme.txt

Sync with the readme.txt from BuddyPress 2.0.2, and fix up old and outdated links in the plugin description that appear prominently on wordpress.org/plugins/buddypress/.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8957 cdf35c40-ae34-48e0-9cc9-0c9da1808c22