Add a note that Page.cache_key is not used by FeinCMS itself
Add a note that Page.cache_key is not used by FeinCMS itself

Refs #492.

mjl/feincms,matthiask/django-content-editor,michaelkuty/feincms,nickburlett/feincms,feincms/feincms,joshuajonah/feincms,matthiask/feincms2-content,nickburlett/feincms,matthiask/django-content-editor,nickburlett/feincms,matthiask/feincms2-content,michaelkuty/feincms,feincms/feincms,joshuajonah/feincms,matthiask/django-content-editor,michaelkuty/feincms,feincms/feincms,michaelkuty/feincms,matthiask/django-content-editor,mjl/feincms,joshuajonah/feincms,mjl/feincms,nickburlett/feincms,matthiask/feincms2-content,joshuajonah/feincms