Add a glossary / definition of terms that adapts OGC terminology to MapML / Web

[prushforth]

OGC has a highly technical and specific vocabulary, which can be hard to understand. The objective of MapML is to adapt (some) authoritative geospatial standards for use in (authoritative) Web standards. The audience for Web standards (documents) is typically implementers, who may not be familiar with OGC terminology. A glossary that maps terms' definitions in OGC / ISO standards to terms used in MapML would help both sides understand the other's environment and concepts.