Note nightly availability

Author: sfackler

README.md

@@ -7,3 +7,8 @@ The full readme is generated over in [docker-library/docs](https://github.com/do
 See a change merged here that doesn't show up on the Docker Hub yet? Check [the "library/rust" manifest file in the docker-library/official-images repo](https://github.com/docker-library/official-images/blob/master/library/rust), especially [PRs with the "library/rust" label on that repo](https://github.com/docker-library/official-images/labels/library%2Frust). For more information about the official images process, see the [docker-library/official-images readme](https://github.com/docker-library/official-images/blob/master/README.md).
 
 <!-- THIS FILE IS GENERATED BY https://github.com/docker-library/docs/blob/master/generate-repo-stub-readme.sh -->
+
+## Nightly Toolchain
+
+An image tracking the Rust nightly toolchain is available via
+[`rustlang/rust:nightly`](https://hub.docker.com/r/rustlang/rust/).