llvmPackages_git: 2022-01-07 -> 2022-25-07, add README
The new LLVM commit is just before the LLVM 15 fork off.
The readme describing upgrade process so it is easier for others to do.
Co-Authored-By: Dylan Green <Dylan.Green@Obsidian.Systems>
Co-Authored-By: John Ericson <John.Ericson@Obsidian.Systems>
authored by