Draft for accompanying document

We want to create an accompanying document for LIDO v1.1 covering additional information which shouldn't be listed in the schema docs.

At this moment, this includes:

• introductory texts
• listing the following information per element (where applicable):
  • further information
  • data values
  • equivalent elements in other schemas

Cf. https://wiki.dnb.de/pages/viewpage.action?pageId=167039097