Document configuring custom toolchains

This [WORKSPACE](https://github.com/mfarrugi/cargo-raze-example-stdx/blob/master/WORKSPACE#L7) and [custom_rust_toolchain.bzl](https://github.com/mfarrugi/cargo-raze-example-stdx/blob/master/bazel/custom_rust_toolchain.bzl) is an example of setting up toolchains outside of the default one that is part of these rules.

Is the toolchain feature stable enough to update the documentation to recommend this flavor of custom toolchain configuration? Is it possible to configure targets to require different toolchains (eg. targets that are nightly only)?