Added migration section for ChilliCream#8279 (ChilliCream#8280)

Author: glen-84

website/src/docs/hotchocolate/v16/migrating/migrate-from-15-to-16.md

@@ -16,6 +16,10 @@ Things that have been removed or had a change in behavior that may cause your co
 
 The `@skip` and `@include` directives are now disallowed on root subscription fields, as specified in the RFC: [Prevent @skip and @include on root subscription selection set](https://github.com/graphql/graphql-spec/pull/860).
 
+## Deprecation of fields not deprecated in the interface
+
+Deprecating a field now requires the implemented field in the interface to also be deprecated, as specified in the [draft specification](https://spec.graphql.org/draft/#sec-Objects.Type-Validation).
+
 # Deprecations
 
 Things that will continue to function this release, but we encourage you to move away from.