Document that Pidgin can't open an account with Persistence enabled in Read-only mode

reported by user, confirmed by me.

Pidgin fails to load any previous accounts if booting Tails in Read-only mode with Persistence enabled.

Also doesn’t load the defaults accounts. Just comes empty (no nick randomizer neither)

I know is a corner case, but if its not easy we can at least document it.

Feature Branch: bitingbird:doc/8423-bug_pidgin_persist

Original created by @emmapeel on 8423 (Redmine)

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