RFC: docs: Update licensing information

This updates information on trademark owners, as well as Mesa's current status as far as conformance. In this, the arrangement whereby which Mesa gains the legal ability to undergo conformance testing and use Khronos trademarks when the corresponding hardware vendors pay licensing fees is cited. Also mentioned is how software implementations (such as llvmpipe) must be stated as non-conformant, as they have no corresponding hardware vendor.

This is all based on what I have been able to interpret based on responses to inquiries, and I may be very wrong.

I am grateful to those on irc who helped me begin to understand this legal arrangement, and thank you for any reply.

/cc @eric @evelikov @jekstrand