Rustc-dev-guide: Document Pattern and Exhaustiveness checking

[mark-i-m]

We added a skeleton chapter in #651.

cc @Centril

[Centril]

cc @arielb1 @varkor @Nadrieril

[Nadrieril]

How detailed does that guide aim to be ? Specifically, should we move there the in-depth explanation of the exhaustiveness algorithm that is currently at the top of _match.rs ? Or is that too detailed ?

[Centril]

I think starting with the broad strokes, mentioning that there is an in-depth explanation, as well as linking to the paper would be a good start.

[mark-i-m]

Generally, it's a bit of judgement call. The goal is just for the guide to be a reasonable starting point for being able to contribute. Generally, that means discussion of important design decisions, data structures, routines, or algorithms, as you see fit.

Another goal is that it should avoid mentioning details that are likely to change, and link to the rustdocs for the compiler as much as possible so that we can detect changes.

[Nadrieril]

I'm finally getting around to doing this!