README.rst: formatting + link to readthedoc

I have added an entry for https://servermon.readthedocs.org/ a service
which build Sphinx documentation for us and provide web hosting for
anyone to look at the doc.

While at it, also used code-block for the shell snippets.
Merge pull request #11 from hashar/readme-tweak-and-readthedocs

README.rst: formatting + link to readthedoc