Update to docs/README.html

The current docs/README.html requires updating.

This relates to http://tracker.phpbb.com/browse/PHPBB3-9896, but is a more serious issue because the information related to language packs, not only the link, is out of date and incorrect, so the README.html is actually "broken".

Have made some changes to the information related to Styles, updated some links and included a reference to the Knowledge Base.

Rather than explaining everything that's changed I've just included an updated version.