Improve, clarify, and expand documentation

Further improvements are possible:  delete todo.txt after creating any
extant issues, and update/backfill the changelog.  The license also
needs to be clarified; the text simply says GPL and the link goes to v1
- I think "your choice of version" would be an appropriate
interpretation.
Merge pull request #30 from PeridexisErrant/improve-documentation

Improve, clarify, and expand documentation