Move the MFC doc from my freebsd-git-docs repo to handbook

Integreate MFC procedures into the committer's handbook. This was originally in
my freebsd-git-docs repo where it was collaberatively created. The changes here
are light edits and a conversion from markdown to asciidoc.

Contributions from: jhb@, lwhsu@, delphij@, Ceri Davies, Alexandre