Adds changelog entries for v3.13.0 (#824)

* adds changelog entries for 3.13.0

* formats

* Update CHANGELOG.md

Co-authored-by: Sam Stern <samstern@google.com>

* Update CHANGELOG.md

Co-authored-by: Sam Stern <samstern@google.com>

* Update CHANGELOG.md

Co-authored-by: Sam Stern <samstern@google.com>

* formats

* formats

Co-authored-by: Sam Stern <samstern@google.com>

Author: joehan

CHANGELOG.md

@@ -0,0 +1,18 @@
+- Adds `serviceAccount` option to `runtimeOptions` to specify which service account Cloud Function should use at runtime. For example:
+
+```
+const functions = require('firebase-functions');
+
+exports.myFunction = functions.runWith({
+    serviceAccount: 'test-sa@project.iam.gserviceaccount.com'
+    // OR
+    // serviceAcount: 'test-sa@"
+    // OR
+    // serviceAccount: 'default'
+  })
+
+```
+
+Requires firebase-tools@8.18.0 or later. Thanks @egor-miasnikov!
+
+- Upgrades `highlight.js` to `10.4.1` to fix a vulnerability.