Auto merge of #3147 - carols10cents:versions-in-readme, r=alexcrichton

Add information about Cargo releases going with Rust releases

Closes #3101.

This just adds a table of rust release numbers to cargo release numbers, and some text that clarifies that they happen together.

I put the table behind a details tag, which Chrome renders really nicely with a little toggle triangle... Firefox will do the toggling *functionality* but doesn't have much of an indication that you can click on the text, but I think they're working on it. Servo has a little triangle but nothing happens when you click on it ;)