"Usage" section of the README.md forgets to show where `RBush` comes from

[Pomax]

The usage section currently reads:

Creating a Tree
const tree = new RBush();

But it's not clear where RBush comes from. Is this a const RBush = require("rbush");? Or is it the older const rbush = require("rbush"); const RBush = rbush.RBush;? or....?

Can the readme be updated to include the missing, but crucial, loading code?

[Pomax]

nice, thanks!