Add information to describe possible error

[software-opal]

I've spent the last few hours trying to get a CKAN instance running and finally fixed it by applying the patch.

Hopefully adding this will save someone else many hours of debugging.

[anotheredward]

Hi @leesdolphin, thanks for your PR! When you say "then you have not installed the patch to CKAN correctly." What did you have to change to get the plugin working correctly? Thanks :).

[camfindlay]

This is referring to the middleware patch on CKAN core I believe. It is in the documentation https://github.com/data-govt-nz/ckanext-security#requirements

[anotheredward]

LGTM