First pass of improving the documentation for English speakers

It's the first pass. Please send me feedback.

There still remains:

 *  Fixing the "use case" issue, which means something else (see IRC)
 *  Adding a section explaining the background, the terms, purpose, an overview of its performances, etc.
 *  Adding a section explaining how LeoFS works, what guarantees it gives, etc.
 *  Consolidating some of the sections which repeat themselves a bit

I'll work on all of them in that order.