iXhash plugin docs updated, version for 3.0.x added.

[Dirk Bonengel]

Hi list,

just as the subject says: I added some stuff to 
http://wiki.apache.org/spamassassin/iXhash and
- made it clear (I hope) what it actually does
- added installation instructions
- added a version that runs under SpamAssassin 3.0.x
Dirk