Update README with documentation link
Update README with documentation link

Remove duplicate documentation from the README.
Add link to Sphinx docs on ReadTheDocs.

The duplicate documentation was the inspiration (and sometimes
copy-paste source) for much of the Sphinx documentation. No
documentation was lost.

nojhan/liquidprompt,laurentbardin/liquidprompt,milouse/liquidprompt,pkkolos/liquidprompt,jaesivsm/liquidprompt