htdocs/news/status: Update README

Update README with information about the mailing list
freebsd-quarterly-calls@ and the sendcalls perl script.

Also warn about the md2docbook perl script becoming obsolete once the
Hugo/AsciiiDoctor conversion is done.

Reviewed by: debdrup
Approved by: docs (bcr), gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D25193