Update CHANGELOG.md

Author: kriswest

CHANGELOG.md

@@ -9,6 +9,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 ### Added
 
 * Added clarification that `id` field values SHOULD always be strings to context schema definition (a restriction that can't easily be represented in the generated types). ([#1149](https://github.com/finos/FDC3/pull/1149))
+* Added requirement that Standard versions SHOULD avoid the use unions in context and API definitions wherever possible as these can be hard 
+ to replicate and MUST avoid unions of primitive types as these can be impossible to replicate in other languages. ([#120](https://github.com/finos/FDC3/pull/1200))
 
 ### Changed