Add changelog entry

Author: oliverklee

CHANGELOG.md

@@ -21,6 +21,7 @@ Please also have a look at our
 
 ### Changed
 
+- Only allow `string` for some `OutputFormat` properties (#885)
 - Make all non-private properties `@internal` (#886)
 - Use more native type declarations and strict mode
   (#641, #772, #774, #778, #804, #841, #873, #875)