Word-wrap the README

README.md is rather hard to read in a text editor since it's not
word-wrapped.  This commit word-wraps it.
Fix Markdown style

To make README.md even easier to read in a terminal, I replaced some
literal HTML formatting with the equivalent Markdown syntax.  In
particular I moved some URLs away from the location where they were
being linked to.
Merge pull request #263 from kini/readme

Make README.md easier to read as plain text