Document how to find the nightly version that a beta/stable was branched from

[jyn514]

this is what I have so far, might be something more reliable. doesn't work before ~1.65.

; rustc_version_bump=$(git log --format=%H -n1 -G CURRENT_RUSTC_VERSION origin/beta -- compiler/rustc_feature/src/accepted.rs)
; git log --format=%H -n1 --author=bors $rustc_version_bump