Add warning about MRI image redaction/confidentiality.

I thought it would be a good idea to add a warning about image redaction for MRI protocols.

The way I see it, the best (maybe only) way to guarantee confidentiality is if the original images are edited to black out identifiable features, and saved in a format which does not support layers, or preserve edit history. We could use this pull request as a discussion point if anyone has further thoughts.