Add the rustc guide to the cheatsheet

Author: phansch

doc/adding_lints.md

@@ -399,6 +399,7 @@ Here are some pointers to things you are likely going to need for every lint:
 * [`in_macro`][in_macro] and [`in_external_macro`][in_external_macro]
 * [`Span`][span]
 * [`Applicability`][applicability]
+* [The rustc guide][rustc_guide] explains a lot of internal compiler concepts.
 
 For `EarlyLintPass` lints:
 
@@ -437,3 +438,4 @@ don't hesitate to ask on Discord, IRC or in the issue/PR.
 [in_external_macro]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc/lint/fn.in_external_macro.html
 [play]: https://play.rust-lang.org
 [author_example]: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f093b986e80ad62f3b67a1f24f5e66e2
+[rustc_guide]: https://rust-lang.github.io/rustc-guide/