Add project roadmap to the repository

[tsalo]

Summary

In today's NiPreps roundup, one of the agenda items was requesting project roadmaps on nipreps repos.

[tsalo]

A few important elements:

1. Get surface projection working (Get surface projection working #297)
2. Remove internal copy of SDCFlows and update to most recent version (Remove external dependencies #199)
3. Add AtlasPack atlases (Add AtlasPack atlases #330).

[tsalo]

Since I've actually addressed all of the items in the above list, I'm not sure what the roadmap should look like now.