New README.md for llvm_version.txt

[rengolin]

Fixes #936

[adam-smnk]

Looks good, thanks

[rolfmorel]

Thanks for the explanation and documenting it in-repo, @rengolin!

It will take some time to become acquainted with the development experience laid out in this document (e.g. I am new to git worktree). All the more, thanks for spelling it out.

(When working this way, I'll try to see if I can address the question for myself if saving the commit id in a .txt is really that different from just having it in a .gitmodules file. That is, as far as I understand, a git submodule does not commit you to certain workflows. You could just use it to document a current designated commit for some other project somewhere else (though it has a default location according to .gitmodules) . Maybe there's too much baggage associated with git submodules to go this route anyway.)

[rengolin]

FYI, we're not the only ones using this method. StableHLO does a similar thing.