message: Update CHANGELOG.md
  * Update CHANGELOG.md
  Added environment variable search path to changelog
  * Move new feature above build updates
diff: - Added the MaterialX\:\:GeomPropDef class for geometric input declarations.
  - Added the Document\:\:getGeomAttrValue method.
  - Added the ValueElement\:\:getResolvedValue method.
  +- Added support for setting search path via environment variable.
  - Added support for GCC 8 and Clang 7.
  ### Changed
  