docs(changelog): add changelog entry for Nullable additions

Author: glennsl

CHANGELOG.md

@@ -14,6 +14,7 @@
 - Add locale and formatting options to `localeDateString`, `localeString` and `localTimeString` functions https://github.com/rescript-association/rescript-core/pull/30
 - Change `RegExp.source` to return a `string`. Was previously returning a `bool`, which is wrong. https://github.com/rescript-association/rescript-core/pull/47
 - Remove `Date.valueOf` as it returns the same as `Date.getTime`. https://github.com/rescript-association/rescript-core/pull/61
+- Add `getExn`, `getUnsafe`, `getWithDefault`, `map`, `mapWithDefault` and `flatMap` to `Nullable`. https://github.com/rescript-association/rescript-core/pull/67
 
 ### Documentation