Add changelog entry for null safety (#126)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 64d6cd5..e05854e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 ## 0.17.0-nullsafety.0
 
+- Migrate to null safety.
 - `Font.merge` and `BoxEdge.merge` are now static methods instead of factory
   constructors.
 - Add a type on the `identList` argument to `TokenKind.matchList`.