[DFDL-WG] updates to experimental features convention document: enums

Updates to allow enumeration properties to be extended with new experimental enum values. See https://github.com/OpenGridForum/DFDL/pull/32 Mike Beckerle Apache Daffodil PMC | daffodil.apache.org OGF DFDL Workgroup Co-Chair | www.ogf.org/ogf/doku.php/standards/dfdl/dfdl Owl Cyber Defense | www.owlcyberdefense.com