Update links in docs/readme.html

For section 2 it links to http://www.phpbb.com/mods/ and http://www.phpbb.com/styles/ for links to find styles and mods when it might be better to link straight to the DB.

Same for languages linking to the downloads page.

Also http://www.phpbb.com/ under community forums (3.ii) could be linked directly to the forums at /community/