Document changes in UPGRADING.md

Author: MatteoPologruto

docs/UPGRADING.md

@@ -2,6 +2,12 @@
 
 Here you can find a list of migration guides to handle breaking changes between releases of the CLI.
 
+## 0.31.0
+
+### Added `post_install` script support for tools
+
+The `post_install` script now runs when a tool is correctly installed and the CLI is in "interactive" mode. This behavior can be [configured](https://arduino.github.io/arduino-cli/0.30/commands/arduino-cli_core_install/#options).
+
 ## 0.30.0
 
 ### Sketch name validation