This file doesn't need to be created initially, as it will be done
automatically during the build, but it does need to be updated whenever
setup0.h changes, so explain this in both the README-GIT.md file and the
main build instructions.
Closes#18699.
Rename BuildGit.txt to README-GIT.md, which is a more standard name, and
a better format for viewing this file online, e.g. from GitHub Web UI,
and link it from the main README to make it easier to find.