Note bone-hand events in related example for visibility

[Alexander-Krause-Glau]

I suggest to write down the two emitted events (handMeshCreated, handMeshUsed) of the bone-hand plugin in the related example. This is also done for the rigged-hands example. I wasn't aware they exist and tried stupid stuff for obtaining all hand meshes from a three.js scene.

For instance, I use this for frustum culling, e.g. hide the Leap Hands when Vive controllers are in sight.

A pull request seems inappropriate to me. However I will create one if requested. Thanks for this incredible plugin.