Updated Plugins Guide - questions and RFT

[DeeJay]

re: loggerhead. Description should be "Web inteRface for browsing branches"

It would be helpful to note that this plugin is not useable by Windows
users running from the standalone bzr.exe installer.

In general for the whole guide: I can't see 'how to install' or 'where
to download' sections for any of the plugins - should they be here or
are they a topic for another webpage? And would it be useful to list
pre-requisites?

Thanks
DJ