Add comment about blocked images

Fixes https://github.com/PrivateBin/PrivateBin/issues/275

cfg/conf.sample.php

@@ -69,6 +69,7 @@ languageselection = false
 ; scripts or run your site behind certain DDoS-protection services.
 ; Check the documentation at https://content-security-policy.com/
 ; Note: If you use a bootstrap theme, you can remove the allow-popups from the sandbox restrictions.
+; By default this disallows to load images from third-party servers, e.g. when they are embedded in pastes. If you wish to allow that, you can adjust the policy here. See https://github.com/PrivateBin/PrivateBin/wiki/FAQ#why-does-not-it-load-embedded-images for details.
 ; cspheader = "default-src 'none'; manifest-src 'self'; connect-src *; script-src 'self'; style-src 'self'; font-src 'self'; img-src 'self' data:; media-src data:; object-src data:; Referrer-Policy: 'no-referrer'; sandbox allow-same-origin allow-scripts allow-forms allow-popups"
 
 ; stay compatible with PrivateBin Alpha 0.19, less secure