reorder README sections
minor README content changes
README: add more description to some backends

Give a bit more detail on the complex ones.
README: Minor text tweaks
Merge rearrange-readme-sections (#4) into master

Update README

I was reading through the documentation, and made some adjustments that
I thought would have helped me grasp it faster.  Among other things,
sections in the README were ordered alphabetically which meant `Scope`
was located after `MultiScope`. This required a lot of jumping around
when reading from top to bottom.