Add initial Roadmap draft

[jamesmunns]

As requested by ISRG, I've prepared a first draft of a feature-focused roadmap towards a 1.0 release.

[jamesmunns]

Note:

• Add a sentence wrt: prioritization for each marquee feature
• "Reason for importance for each major feature".

[jamesmunns]

Note:

• Pingora is interested in caching in the near future (next 6mo), but likely only the interface
• Pingora is interested in H3, based on prioritization

[jamesmunns]

This was reviewed at the biweekly status meeting, and I'll merge now. Open to feedback in the future!