gh-37458: Improve FLINT autogen README
The FLINT headers are generated manually using a script from #36449.
However, the documentations are not quite clear, and neither is the
commit that the current headers are generated from. This commit adds
these two details.
@videlec do you mind reviewing? I am not sure if the `flint-commit.txt`
is an acceptable way to put this information, or where I should put it.
Also I'm aware that this script might be moved in the future (since it
shouldn't belong in `sage_setup`?) but that shouldn't conflict with this
PR.
URL: https://github.com/sagemath/sage/pull/37458
Reported by: grhkm21
Reviewer(s): Vincent Delecroix
