add warning for csp exceptions

I found it notable to point out that XSS on one path gives DOM/XMLHttpRequest access to all other paths.
CSP exceptions sound like a bad idea to me, but might still be justifiable on secure subpages which underwent a higher level of scrutiny.

I hope this is valid sphinx syntax and shows a warning box. My knowledge in this area is limited.. :)
changed wording of csp exception warning
Merge pull request #31 from freddyb/patch-1

add warning for csp exceptions