chore: updated UPGRADING.md

Author: Luca Bianconi

Diff for: docs/UPGRADING.md

@@ -4,6 +4,12 @@ Here you can find a list of migration guides to handle breaking changes between
 
 ## 0.30.0
 
+### Sketch name validation
+
+The sketch name submitted via the `sketch new` command of the CLI or the gRPC command `cc.arduino.cli.commands.v1.NewSketch` are now validated. The applied rules follow the [sketch specifications](https://arduino.github.io/arduino-cli/dev/sketch-specification).
+
+Existing sketch names that don't respect the new constraints need to be updated.
+
 ### `board attach` CLI command changed behaviour
 
 The `board attach` CLI command has changed behaviour: now it just pick whatever port and FQBN is passed as parameter and