Document how to enable send2cgeo when third-party cookies are disabled

[SnarkBoojum]

When third-party cookies are disabled, then an exception for https://send2.cgeo.org needs to be added ; that should be documented somewhere (and probably with instructions for the various browers on how to do that).

[Lineflyer]

We have this very basic explanation in our FAQ already:
http://www.cgeo.org/faq#send2-register-first

Is this sufficient or which additional information would you suggest?

[SnarkBoojum]

I haven't found this when I looked into the problem :-/