Mention that lots of irc servers block Tor

Originally created by @BitingBird on #9398 (Redmine)

It’s a very, very common question on irc. If we made a small note in https://tails.boum.org/doc/anonymous_internet/pidgin/ it could make our life easier :)

My common answer is: Lots of servers block Tor, because of spambots. Most irc servers have a webpage where they explain what their policy is. Freenode in particular says they have a .onion but it’s down most of the time.

Feature Branch: doc/9398-irc-blocking-tor