docs(changelog): add changelog entry for Nullable additions

Author: glennsl

CHANGELOG.md

@@ -17,6 +17,7 @@
 - Remove `Date.valueOf` as it returns the same as `Date.getTime`. https://github.com/rescript-association/rescript-core/pull/61
 - Change `Float.fromString` signature. Now accepts only string. https://github.com/rescript-association/rescript-core/pull/54
 - Change `Float.parseFloat` signature. Now accepts only string. https://github.com/rescript-association/rescript-core/pull/54
+- Add `getExn`, `getUnsafe`, `getWithDefault`, `map`, `mapWithDefault` and `flatMap` to `Nullable`. https://github.com/rescript-association/rescript-core/pull/67
 
 ### Documentation