Add a pointer to the more comprehensive design doc.
# J2CL JsInterop by Example
-This is meant to be a quick guide on how to use JsInterop.
+This is meant to be a quick guide on how to use JsInterop. More details can be
+found in the
+[design doc](https://docs.google.com/document/d/10fmlEYIHcyead_4R1S5wKGs1t2I7Fnp_PaNaa7XTEk0/edit?usp=sharing).
## Calling JavaScript from Java
