Minor typo fixes, and big changes to README

- A few typo fixes in the README 
- `s/those/these/` in `pref_expert_options_summary`. Still sounds oddly phrased to me, but I think this is better gramatically.

Before:

    Please be careful with those

After:

    Please be careful with these

I also just pushed a third commit where I started to make parts of the README more readable in plaintext and ended up going a bit wild; fixing typos, grammar, rewrapping long lines, fixing poorly phrased sentences, punctuation, etc. Content remains largely the same.