Several bugs in installation documentation

[Zagrebelin]

I just create empty virtualenv, make pip install django-kb & djangoadmin startproject and can't start site.

1. INSTALLED_APPS need "tagging" application
2. Without markdown installed module got this error while manage.py syncdb
3. admin site want HAYSTACK_CONNECTIONS setting.

[eliostvs]

I will update the docs, meanwhile you can see an example of the settings here. Thanks!