add readme for this repository

Author: pietroalbini

README.md

@@ -0,0 +1,9 @@
+# `ferrocene/rust`
+
+This repository is the fork of [`rust-lang/rust`] we use at Ferrocene to
+upstream the changes we make to the Rust toolchain. This is **not** the source
+code of the [Ferrocene toolchain][ferrocene], which is hosted in a private
+repository.
+
+[`rust-lang/rust`]: https://github.com/rust-lang/rust
+[ferrocene]: https://www.ferrocene.dev