atf/FREEBSD-upgrade: update document/rewrite in Markdown

[ngie]

Summary

The document does not reflect current reality with how updates/releases
should be managed with contrib/atf . Update it to a state that better
reflects current practices with git, as well as information about where
the latest version of the project can be found, etc.

This change will be used as a template for adding instructions on how to
update contrib/kyua and contrib/lutok in future commits.

MFC after: 1 month

Test Plan

I confirmed that the rendered version looks good on GitHub proper: https://gist.github.com/ngie-eign/8a3614304aaaab12090e5be0900dae36

[ngie]

Minor tweaking to the "Contributing Changes" section

[ziaee]

The link in the test plan seems out of date?

[ngie]

In D59879#1374256, @ziaee wrote:

The link in the test plan seems out of date?

It's public now (I forgot that public GH only supports public/private :/..).