Update CHANGELOG.md

CHANGELOG.md

@@ -5,7 +5,10 @@ All notable changes to this project will be documented in this file.
 ## [1.20.0] – 2021-07-16
 
 ### New
-- ABI version `2.1` supported.
+- ABI version `2.1` supported.  
+  **Attention!**
+  If you work with contracts, that contain String parameters, then during migration from ABI 2.0 to 2.1 you will need to remove all String type conversions to bytes and back and pass string to your contract as is.  
+
 - Now all requests to GraphQL are limited with timeout to react on unexpected server unavailability.
 Existing timeouts in waiting functions keep the same behaviour. All other requests timeout now can 
 be set with `net.query_timeout` config parameter. Its default value is 60000 ms