Readme.md: Assorted cleanups (reference-style links, typos, etc.)

This fixes a few of the more obvious issues I had with the README.
Grepping turned up some duplication between `Readme.md` and
`doc/source/tutorial.rst`.  It's probably better to remove the
tutorial from the README and just link from the README to a compiled
version of the tutorial.  There are also a number of relative paths in
the README that I think should be links to the GitHub repository.  But
that is more than I have time for now ;).