Minor improvements in README.rst
Minor improvements in README.rst

- Add link to doc.wikimedia.org for further documentation
  in a more visible place.
- Add commands to install pywikibot via PyPI

Change-Id: Ica65a16befbe1a6fe7471a3ac44c5ca7ae5fe135

darthbhyrava/pywikibot-local,hasteur/g13bot_tools_new,wikimedia/pywikibot-core,VcamX/pywikibot-core,h4ck3rm1k3/pywikibot-core,Darkdadaah/pywikibot-core,trishnaguha/pywikibot-core,valhallasw/pywikibot-core,npdoty/pywikibot,jayvdb/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,Darkdadaah/pywikibot-core,h4ck3rm1k3/pywikibot-core,happy5214/pywikibot-core,jayvdb/pywikibot-core,happy5214/pywikibot-core,TridevGuha/pywikibot-core,magul/pywikibot-core,hasteur/g13bot_tools_new,smalyshev/pywikibot-core,icyflame/batman,PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,npdoty/pywikibot