Update README ready for version 0.1
Update README ready for version 0.1

Remove the notice that the software is incomplete since it's now
functional as an MVP.

Link to the GitHub Milestones page which will act as living
documentation of the project roadmap.

The TODO list has moved to the GitHub issues page:
https://github.com/mattbostock/leavediary/issues

mattbostock/leavediary