Add directions for setting up whoosh
See hyperkitty#125 (closed) and https://hyperkitty.readthedocs.io/en/latest/install.html#setting-up-the-databases
Even though hyperkitty's documentation mentions about Whoosh or setting up django-haystack, because it is under Databases
section, I skipped it over because django manages database on its own. Maybe it deserves it own header and a link to the django-haystack documentation to setup whoosh or any other backend that people need.