add website.md page

Brian Paul asked to no longer be listed as the Mesa3D.org webmaster, as
he no longer has the time to maintain it, and the maintainence has
effectively become a community effort anyway.

This page replaces the old webmaster.html page with some more up-to-date
information about the site and how to make changes to it.
2 jobs for master in 20 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #3088181
pages

00:00:20

 
  Deploy
passed pages:deploy

00:00:01