docs(changelog): add 'porting' scope

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: I22a81b3f69d90e0fcb88c7e98178e915253afb43
diff --git a/changelog.yaml b/changelog.yaml
index e100f82..a514abd 100644
--- a/changelog.yaml
+++ b/changelog.yaml
@@ -1219,6 +1219,9 @@
       - title: Threat Model
         scope: threat-model
 
+      - title: Porting Guide
+        scope: porting
+
   - title: Build System
     scope: build