Document keyboard and mouse controls

[manthey]

This should be added somewhere in the repo. Ideally, we would also have a help link on the HistomicsUI page that would go to such a page or the root of appropriate set of pages.

This should include keyboard bindings inherited from GeoJS.

[manthey]

This is partly satisfied by #168. We need to decide how to link to these docs.

[manthey]

I'm going to close this as it is basically satisfied and open a new issue with a proposed help button and link that can be configured.