Auto merge of #43066 - pornel:patch-1, r=Mark-Simulacrum

Suggest rustup toolchain link

The contributing instructions only explain how to build the compiler, but not how to try it out.

I found `rustup toolchain link` to be super useful for this, so I think it's good to suggest it.

Author: bors

CONTRIBUTING.md

@@ -232,6 +232,7 @@ Some common invocations of `x.py` are:
   guidelines as of yet, but basic rules like 4 spaces for indentation and no
   more than 99 characters in a single line should be kept in mind when writing
   code.
+- `rustup toolchain link <name> build/<host-triple>/<stage>` - Use the custom compiler build via [rustup](https://github.com/rust-lang-nursery/rustup.rs#working-with-custom-toolchains-and-local-builds).
 
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