Add info for template context processors.

I couldn't figure out how to add events to the calendar using the
default templates. The calendar_month.html template references
the request variable, which is only available if you add the
django.core.context_processors.request processor to the
TEMPLATE_CONTEXT_PROCESSORS setting.  Doing so will display the
'Create new event' link.
Merge pull request #13 from minadyn/update-install-docs

Add info for template context processors.