(DOC) Add a CONTRIBUTING page

This page provides instructions to community members on how to contribute to the DNN.Platform project.
Fix message in host settings that implies it isn't in the host-level context

Looks like the message about debug mode was copied from site settings, without
changing the reference to "host level"
Merge pull request #11 from bdukes/fix-host-crm-message

DNN-3771 Fix message in host settings that implies it isn't in the host-level context