Add READMEs for the credential helpers.
This adds some READMEs for these crates. These are pretty bare bones for now, but I suspect they may get extended in the future based on how we decide to deploy them.
This is a short-term option until we can have a better solution for
globbing. This does not update `benches/` to support which has a README
in there preventing globbing; this seems low-churn enough not to find a
solution for it.
On the next sync-up with rust-lang/rust, we'll need to update 4e46301258/src/bootstrap/tool.rs (L588-L603)Fixes#11988