Implementation Guidance page added to technical.buildingsmart.org

Reviving this old thread for a note that the guidance section about encoding is broken — some backslashes disappeared: It should read \S\D, \PA\\S\D, \X\C4, and \X2\00C4\X0\. I suggest to include the strings in <pre> or <code> tags. If markdown entry is supported in the authoring backend, use enclosing backticks (`).