Update README to be concise, consistent & accurate

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

[skip ci]