readme: hard-wrap long lines at 72 chars

This makes the readme readable in a normal-sized terminal and has the
extra benefit of making diff output easier to understand on the
command line, in tools like gitg, and on GitHub.
readme: improve readability of headings
readme: copy-editing
readme: write small numbers as words
readme: simplify language about input length limits
Merge pull request #17 from mgeisler/readme-cleanups

Readme cleanups