Document "dev mode"

In `sirv --help` "dev mode" isn't explained, I had to go to the npm package to find
> [...] Instead, when not in "dev" mode, sirv performs all its file-system operations upfront and then relies on its cache for future operations.

I think this should be mentioned in the readme and help text, since a nieve person (me) might think "dev mode" includes some kind of livereload.