Add documentation for extending less with custom functions

This pull request replaces #86. It turns out that a `peerDependency` isn't necessary to get a copy of less at the root of your project that less-middleware also uses. Just use the handy `npm dedupe less` command to consolidate down to one copy at the root level. And if you `npm install` less and less-middleware at the same time, npm will helpfully do the right thing without needing to dedupe.

I think these docs are a good idea since they'll help people (like me) figure out how to extend less with custom functions when using the middleware.