Improve readme

[romulof]

Read it all twice and I still have no idea how this thing works.

In practical terms, Corepack will let you use Yarn and pnpm without having to install them.

How? A usage example would be really helpful.

[styfle]

The readme could use some improvement.

Here's a shameless plug for a blog post I wrote that should answer all your questions:

https://styfle.dev/blog/corepack