Link directly from the README to rustc-dev-guide

[jyn514]

Location

rust/README.md

Lines 10 to 12 in fcb2a36

	**Note: this README is for _users_ rather than _contributors_.**
	If you wish to _contribute_ to the compiler, you should read
	[CONTRIBUTING.md](CONTRIBUTING.md) instead.

rust/CONTRIBUTING.md

Lines 11 to 21 in e6989d2

	Documentation for contributing to the compiler or tooling is located in the [Guide to Rustc
	Development][rustc-dev-guide], commonly known as the [rustc-dev-guide]. Documentation for the
	standard library in the [Standard library developers Guide][std-dev-guide], commonly known as the [std-dev-guide].
	## About the [rustc-dev-guide]
	The [rustc-dev-guide] is meant to help document how rustc –the Rust compiler– works,
	as well as to help new contributors get involved in rustc development. It is recommend
	to read and understand the [rustc-dev-guide] before making a contribution. This guide
	talks about the different bots in the Rust ecosystem, the Rust development tools,
	bootstrapping, the compiler architecture, source code representation, and more.

Summary

Right now, there's a lot of steps involved in finding useful info about how to build the compiler (if you're not following the steps in the README, which are intended for distros). For example, to find how to test the standard library, you'd have to go from the README -> CONTRIBUTING -> https://rustc-dev-guide.rust-lang.org/ -> notice "Testing the compiler" in the sidebar and realize it might be relevant to the standard library too. We should improve several of these steps, but the easiest one is to link directly to the rustc-dev-guide from the README.

Unfortunately, CONTRIBUTING.md still has some information that's not on the top-level "Getting Started" page in the dev-guide; in particular, rustc-dev-guide doesn't link to std-dev-guide until way down in "other places to find information": https://rustc-dev-guide.rust-lang.org/about-this-guide.html?highlight=std-dev-guide#other-places-to-find-information. We should make this more prominent, and then we can update the README to link directly.

cc https://rust-lang.zulipchat.com/#narrow/stream/122652-new-members/topic/testing.20library.2Fcore

[shibcreate]

@rustbot claim