recast-navigation-three README should mention await init() needed

[rob-myers]

Hey, thanks for the awesome work.

I couldn't get @recast-navigation/three to work without first importing init from recast-navigation and bootstrapping e.g.

init().then(() => { ... })

If this is expected, it would help to include this step in the README.

[isaac-mason]

Thanks! Yep this is a documentation miss. I've improved the README now: 4de57c0

Let me know if you've got any other suggestions, much appreciated.