README: See https://github.com/vandry/mairix for maintenance of mairix

Per discussion on the mailing list here:

https://sourceforge.net/p/mairix/mailman/message/35929900/

(unilateral discussion, unfortunately), this project can be considered orphaned. Please see my fork for the latest version instead, and please open new issues, if you'd like to raise any, on my fork.

https://github.com/vandry/mairix