Document how to configure private browsing mode access for study add-ons

[motin]

We should make that the example study and template take this important aspect into account, relevant from Fx 67:

Privileged extensions automatically have private browsing, unless they opt-out in the manifest (incognito: not_allowed)

A normally signed extension would have to have pbm turned on by a user. A mozilla-signed extension gets it automatically, but can set not_allowed (in which case the user cannot turn on pbm)

Thanks @mixedpuppy for pointing this out!

[mixedpuppy]

I should be a touch more explicit.

Normally signed:
manifest.incognito = spanning : extension can gain access to pbm via user choice
manifest.incognito = not_allowed: extension can never have pbm access

Mozilla signed
manifest.incognito = spanning : extension has access to pbm, cannot be turned off by user (I think, but that may be limited to builtin addons)
manifest.incognito = not_allowed: extension can never have pbm access