Add docs for Yarn PnP usage

[johnnyreilly]

This is a reminder for me to add docs to the project detailing how you can use fork-ts-checker-webpack-plugin with Yarn PnP. (See: arcanis/pnp-webpack-plugin#10 (comment) )

I've written a blog post: https://blog.johnnyreilly.com/2019/06/typescript-webpack-you-down-with-pnp.html

For now I've just put a link in the fork-ts-checker-webpack-plugin GitHub page to that post. But a docs addition would be clearer / better.

[piotr-oles]

Solved in the alpha release :)