Text and language editing of release-notes.md

Rewrapped text (which was mangled a bit in email); made small language changes for readability, punctuation, and such. Did *not* touch anything after the subhead "0.9.0rc2 Release notes". This is the first contribution by me (tgeller); see https://sourceforge.net/p/bitcoin/mailman/message/32046281/ to understand why I did it as I did, and please correct me if I've done anything wrong. Thanks!