Update project.json

Author: samtstern

.opensource/project.json

@@ -16,7 +16,8 @@
         "firestore/README.md": "Cloud Firestore",
         "docs/upgrade-to-2.0.md": "Upgrade to v2.0",
         "docs/upgrade-to-3.0.md": "Upgrade to v3.0",
-        "docs/upgrade-to-4.0.md": "Upgrade to v4.0"
+        "docs/upgrade-to-4.0.md": "Upgrade to v4.0",
+        "docs/upgrade-to-5.0.md": "Upgrade to v5.0"
     },
 
     "related": [