Add some information about compile time performance to the readme

[soenkehahn]

One thing I heard about this library is that it can increase compile times by a lot. I think it'd be helpful to have some information in the readme about that.

Some questions that could be answered:

• How much worse are compile times?
• Are there patterns that can be avoided to speed up compilation?
• Is it better with -O0?
• Is it better with newer ghc versions?

(I read through #110 and that provides some information. But it would be nice to have it a bit more concrete and in the readme.)

Thanks for the awesome library!