Update CHANGELOG.md

CHANGELOG.md

@@ -1,6 +1,12 @@
 
 # Changes to the ATT&CK/STIX Data Model
 
+## 25 April 2023 - ATT&CK Spec v3.1.0
+
+Changes to ATT&CK in STIX for April 2023 ATT&CK Content Release (ATT&CK v13.0)
+
+* Restored the `labels` property for ICS mitigation objects. This property documents security controls for ICS mitigations.
+
 ## 25 October 2022 - ATT&CK Spec v3.0.0
 
 Changes to ATT&CK in STIX for October 2022 ATT&CK Content Release (ATT&CK-v12.0)
@@ -93,4 +99,4 @@ Changes to ATT&CK in STIX for October 2018 ATT&CK Content Release (ATT&CK-v3.0)
 10. Changed ===Windows=== subheadings to ### Windows subheadings (Windows is just one example)
 11. Added space between asterisks (ex. *Content to * Content) to populate markdown correctly
 12. Changed "true" to True in `x_mitre_deprecated`
-13. Added old ATT&CK IDs to Mobile/PRE-ATT&CK objects whose IDs have changed as `x-mitre-old-attack-id`
+13. Added old ATT&CK IDs to Mobile/PRE-ATT&CK objects whose IDs have changed as `x-mitre-old-attack-id`