Update README.md

* Normalized casing of "natural assertions"
* Changed numbered lists which weren't really ordered to unordered lists
* Corrected a few typos
* Corrected a few incorrect statements (I think!)
* Clarified some grammar

Jim - When making the list of what I did, I realized it's an unfocussed commit, parts of which you may agree with more than others. I will be pleased to break it up if you'd prefer. Let me know!