Improvements to readme.txt language

Oxford commas ftw

Fixes #5264

Props hanni

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7605 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Add hanni to 1.9 contributor list

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7606 cdf35c40-ae34-48e0-9cc9-0c9da1808c22