docs: add basic rustup doc

Signed-off-by: MarcoIeni <[email protected]>

terragrunt/modules/rustup/README.md

@@ -0,0 +1,8 @@
+# Rustup
+
+An S3 bucket called `rustup-build` for storing the Rustup binaries,
+distributed through CloudFront CDN.
+
+The [rust-lang/rustup](https://github.com/rust-lang/rustup)
+GitHub repository builds and uploads Rustup artifacts
+to this S3 bucket.