data/ directory README

Layout
------
Each directory
 - must contain exactly one EXPRESS schema
     -- this schema must be useful to end users - i.e. not a schema crafted to test SCL
 - must be named after the numeric designation of the schema *if and only if*
   it is an IS STEP schema
     -- examples: ap214e3, AP235, etc (case insensitive)
     -- directories for schemas that are not STEP IS must be named differently
 - may contain part 21 files
     -- these files must be compatible with the schema
