Document new .onAny() / .onError() methods

[gr2m]

follow up to #324

We deprecated .on("*", handler) and .on("error", handler) in favor of new .onAny/.onError methods, but forgot to document the new methods in the README

I really want to build a CI integration that checks if documentation was updated when we apply a feature label

[gr2m]

As part of that, let's remove documentation for special events: https://github.com/octokit/webhooks.js/#special-events, and update the usage example at https://github.com/octokit/webhooks.js/#usage