Error handling docs

[e-dant]

The messaging syntax (including errors) should be detailed in the readme.

Messages come through event. The where field looks like this:

[s, e]/[sys, self]/[description]

Where s means "successful" (such as s/self/live@path) and e means "error" (such as e/sys/read).

self means we failed. sys means the operating system failed.

The description is usually a function name, an api, or a path. It's something extra.

These events always have the kind field as watcher (as opposed to file, dir, etc).

The what field varies. When a watcher first starts watching, what is create. When a watcher dies, it's destroy. Error messages are usually other.

We need to put this in the readme.

[e-dant]

This could go in the "The Library" section of the readme.

Can't think of a better first issue for potential contributors to get their name up on the project!