Add directory for the userguide and beginning of style guide for it.

git-svn-id: svn://bind10.isc.org/svn/bind10/branches/parkinglot@508 e5f2f494-b856-4b98-b285-d166d9295462

doc/userguide/STYLEGUIDE

+This document will cover rules and suggestions for writing
+and editing the BIND 10 users guide.
+
+Use alphabetical order for options and configuration syntax
+sorting unless you have a specific reason no too.
+
+TODO: list the docbook tags to be used for specific things here