Docs for the postinstall script of the prisma2 npm package

[nikolasburk]

We should document what the postinstall script does when people run npm i -g prisma2 and why it's necessary.

Some context from the public Prisma Slack:

[steebchen]

@timsuchanek Why do we need to fetch the prisma binaries at install time? Isn't it fetched when running commands such as prisma2 dev or prisma2 generate anyway?

[nikolasburk]

I've added the docs here: https://github.com/prisma/prisma2/blob/master/docs/prisma2-cli.md#the-postinstall-hook