Update README to be concise, consistent & accurate

:construction_worker: 

In an effort to create a more useable README, the following has been adjusted:

- [ ] use consistent markdown header syntax of #, ##, ###, ####, etc.
- [ ] make section headers semantically correct
- [ ] adjust language to be more consistent and readable
- [ ] wrap most content at 80 columns for readability and consistency
- [ ] move some content, like the roadmap, to the GitHub wiki
- [ ] update content to be accurate and up to date with the project
- [ ] remove the "to document" section
- [ ] add table of contents to the README