[CMake] [REMINDER] CMake transition to discourse

[Robert Maynard]

Reminder the CMake Discourse forum (   https://discourse.cmake.org )
is the preferred location for CMake questions and discussions. The
current mailman-based mailing lists will be disabled in April 2020,
and their archives will remain available after that.

Reminder for those who prefer email over web forums, the Forum Help
page includes instructions to participate in the forum purely via
email.  Creating topics in the forum via email (and receiving replies)
is supported *with or without* registering a user account.