add readme for vx/hierarchy

[trainorpj]

I have a few notes/questions:

1. It appears that className is passed as a prop to both components, but is never actually called, so I didn't include it in the props table. Am I missing something here?
2. The props size, nodeSize, and separation are all passed to the respective d3 layouts, and that's where the defaults are being set (not in the vx components). Should we mention this, or is it not a big deal?
3. Should this readme include more details on nodeComponent and linkComponent? Is there a plan to have defaults for these, like in vx/network?

Link to components for convenience.

Let me know if anything needs clarification!

[hshoff]

Awesome thank you @trainorpj!

1. This is a bug. Will fix.
2. Also a bug. Will fix.
3. Good catch, should probably default them like @vx/network