`?` key for help

[abitrolly]

Now I need a keyboard shortcuts help. ) Like pressing '?' in GitHub to remind that are the abilities that godu provides beyond fat dir navigation.

[abitrolly]

It is quite easy to add a new key here https://github.com/viktomas/godu/blob/f0108a9f3ae39451b3cae02c7f2d1ad64f3ae0f9/parser.go#L30 and not so easy to understand how draw and toggle help panel with content.

[abitrolly]

There is also a mistake in godu diagram https://github.com/viktomas/godu/blob/master/CONTRIBUTING.md#godu-architecture - no such class as InteractiveTree.

[viktomas]

There is also a mistake in godu diagram https://github.com/viktomas/godu/blob/master/CONTRIBUTING.md#godu-architecture - no such class as InteractiveTree.

Thanks @abitrolly for pointing this out. I've updated the diagram in https://github.com/viktomas/godu/pull/73 🎉