Ticket 340: Include non-normative note about the file-system based implementations of libraries

A library file contains a single library. A library named (A1 A2 AN) is in a file named "A1/A2/AN.scm" (or some other standardized file extension), relative to some "library root" (repository). For portability, library component names should be integers or lower-case identifiers that avoid certain prohibited characters. When a library or top-level imports some other library, the corresponding file is found in the obvious way.