Proposed ports git transition schedule

[Ed Maste]

The doc and src tree git migrations are complete, and the git working
group is preparing the ports tree transition in advance of the 2021Q2
quarterly branch.

The proposed schedule is available at https://wiki.freebsd.org/git and
the status will be updated throughout the process, and important
milestones will be sent here as well.

The committer's guide has been updated with Git documentation, at
https://docs.freebsd.org/en/articles/committers-guide/#git-primer.
Some additional draft / in-progress notes are available in Warner's
repo at https://github.com/bsdimp/freebsd-git-docs/, and additional
documentation will be updated over the next few days.

When the conversion is complete, Subversion will become read-only with
no further updates to the ports tree. If you are fetching ports using
svn, you will need to switch to the new git repository at the
following URLS:

Web repository browser:
    https://cgit.freebsd.org/ports/

Distributed mirrors for anonymous readonly checkout/clone
    https://git.freebsd.org/ports.git
    ssh://anongit@git.freebsd.org/ports.git

Read/write Repository for committers
    ssh://git@gitrepo.freebsd.org/ports.git

We will be monitoring #gitcvt on EFnet (irc.efnet.org) and #git on the
unofficial FreeBSD Discord server (https://discord.gg/WKEKSPtJGp)
during and after the cutover.