Add comment to type declaration file

app/src/frontend/map/react-leaflet-universal.d.ts

@@ -1,3 +1,9 @@
+/**
+ * Export all type declarations available for react-leaflet as types for react-leaflet-universal.
+ * This is because the latter doesn't have type declarations published as of 2019-09-09
+ * but we can re-use types from react-leaflet as universal is mostly a wrapper, so the types
+ * still apply.
+ */
 declare module 'react-leaflet-universal' {
     export * from 'react-leaflet';
 }