Modified: README.rst, added "how to install"

I have written explicit instructions explaining to inexperienced Pelican
users what must be done to use the themes in this repository. Changing 
the theme is the first thing I wanted to do after running the quickstart, 
but it took me a while of googling and reading existing Pelican 
documentation to figure out the (in retrospect, obvious) change I needed 
to make to my pelicanconf.py.