Readme.md: Fix 'constits' -> 'consists' typo
Readme.md: Use reference-style links

Reference-style links [1] make it easier to read the source by moving
the cumbersome URLs out of the main text.  It also isolates the text
from changes in the URLs, which makes for cleaner diffs when URLs
change.

[1]: http://daringfireball.net/projects/markdown/syntax#link
Readme.md: Fix 'Bi as' -> 'Bias' typo

I also wrapped the paragraph to 70 characters (Emacs' default [1])
while I was touching it, since just joing 'Bi' and 'as' produced a
line that was over 200 characters wide.  Wrapping *all* the paragraphs
in the README seemed too invasive for this typo fix, but feel free to
recommend an alternative width or wrapping strategy.

[1]: https://www.gnu.org/software/emacs/manual/html_node/emacs/Fill-Commands.html
Readme.md: Link PyPI references to MurCSS's PyPI page

And use the canonical 'PyPI' capitalization.
Readme.md: Use code literals (`...`) where appropriate

For example, paths, environment variables, and other code should be in
code literals.

I also added a missing period between "The first is located ..." and
"Here is a simple script...".
Readme.md: Fix 'direcory' -> 'directory' typo
Readme.md: Fix 'ensemble_spread' -> 'ensemble spread' typo

Grepping for 'ensemble_spread' turns up no matches in the MurCSS code,
so replace the code-like underscore version with the plain-text spaced
version.
Merge pull request #3 from wking/readme-cleanup

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