[doc] Add AppVeyor Build Status shield and link to README.rst
[doc] Add AppVeyor Build Status shield and link to README.rst

Change-Id: I7f54533a85ef82a94259ef64a72f6975c13820d7

PersianWikipedia/pywikibot-core,wikimedia/pywikibot-core,wikimedia/pywikibot-core