Add a Roadmap
This PR adds a roadmap document. This is useful when pursing funding; we can point to a list of known items that we'd like to do if we had the person time (and funding) to tackle them.

Let's have two discussions

1. Do we want this? Roadmaps tend to go stale. How can we keep this up to date?
2. If so, what items should go on it? I've mostly picked from https://docs.google.com/document/d/151ct8jcZWwh7XStptjbLsda6h2b3C0IuiH_hfZnUA58/edit#heading=h.qm48l6dargmd plus a couple pet peeves of mine.

cc @pandas-dev/pandas-core 