Added unsubscribe instructions to list:member:generic:footer.txt

After witnessing yet another "please unsubscribe me from this list" message sent to a Mailmain mailing list, I decided to propose this change to the generic footer.

I believe this is a sane default, as:

• it allows even the least technically-versed (or most absent-minded) user to immediately reognize how unsubscribe
• it doesn't point to web interface (which some ML don't have)
• the footer is no bigger than before the commit (I shifted $listname on the line above)
• if Listmaster isn't happy, they are free to change it to something else

This should avoid frustration to a bunch of users who are not used to mailing lists.