Update the references to the old Mailman mailing lists to point to Discourse forums.
Reviewed By: aaron.ballman
Differential Revision: https://reviews.llvm.org/D128766
I don't want to rule out the possibility of linking to e.g. interesting
blog posts about uses of Clang, so avoid restricting the content to
"projects".
This breaks URL compatibility, but this document was committed less than
an hour ago so hopefully nobody has linked to it yet.
llvm-svn: 175535