Explain custom middleware registration in the README

[danhodge]

This documentation would have saved me a few minutes the first time I needed to create my own custom middleware.

[iMacTia]

Hi @danhodge thanks for pointing this out.
I agree our documentation is lacking some basic explanation. Good news is that we're going to fix this with the new big release, together with a revamped Wiki.

Appreciating your input, I'm going to close this as it will be part of that revision (see #620).

[mislav]

@danhodge I don't think middleware should necessarily use the register_middleware mechanism. If you define your own custom middleware, you can just use the constant directly:

conn.use MyMiddleware

I think we'll be documenting that approach first and foremost on the wiki.