[doc] Support Libpurple Extensions/Modules/Plugins

Basic Information

Feature: Support libpurple extensions.
Examples: whatsapp-purple, skype4pidgin, libpurple-signald, slack-libpurple, e.t.c.
Why?: Included in most other popular libpurple apps is a UI to manage plugins.
These usually add extra functionality like supporting more chatting protocols.
Solution: This could simply be a documentation issue, considering chatty already included some plugins.
Most plugins already include Debian binaries.

More Information

What is Libpurple?: SuperUser Question; Pidgin Wiki