Null vs Undefined in the docs/readme

[travispulley]

This was easy enough to work around, and probably very pedantic, but the documentation specifies the configuration options often have a default value of "null", but the type definitions indicate "undefined" instead and often don't allow null according to their types.

So there's some conflict with the docs and it caused me about 5 seconds total of confusion.

I propose that it would be helpful to correctly indicate the default and allowed types in the readme / docs.

Thanks for listening! I can draft up a PR if that would help