Preload the README to improve load time of crate pages

[lovasoa]

Here is a lighthouse report for a package page on crates.io :

A time-to-interactive of 8.6 seconds on mobile makes the site difficult to use.

[lovasoa]

I think something to prioritize is to preload the readme. It should be easy to do and bring a big benefit in terms of page load time.

[carols10cents]

Most of this is likely a duplicate of #204, using ember fastboot. Preloading the readme is an interesting, independent idea, so I'm going to leave this issue open for exploring that possibility.