Update Readme.

Author: WaVEV

README.md

@@ -138,6 +138,8 @@ Migrations for 'admin':
 - The `tzinfo` parameter of the `Trunc` database functions doesn't work
   properly because MongoDB converts the result back to UTC.
 
+- In `jsonfield`, when a binary operator (like `exact`) is used, the behavior may differ from other backends if either operand (left-hand or right-hand side) is `null`.
+
 ## Troubleshooting
 
 TODO