Auto merge of #4159 - matklad:arch, r=alexcrichton

Auto merge of #4159 - matklad:arch, r=alexcrichton

Blurb about Cargo inner workings

Hi!

At today's dev tools meetings we've discussed how we can foster contributions to rust dev tools, and I've complained that some tools are difficult to contribute to because it's difficult to learn how they work because there are no docs, and some other people complained that Cargo is difficult to contribute to because it is rather complex. And, as a member of Cargo team, I thought that I am probably responsible for fixing that :)

So, here's my take at making it easier to dive in into Cargo! I've written a small birds eye overview of the current architecture of Cargo (more like a list of things to look at while reading the code actually :)

In general, I am skeptical about documenting internals of binaries (docs will become obsolete, and very fast), but such a high level picture should be pretty robust (I deliberately avoided linking to the actual source code), and so pretty low-effort to maintain. We do something similar for IntelliJ Rust: https://github.com/intellij-rust/intellij-rust/blob/master/ARCHITECTURE.md as well.

r? @alexcrichton