go.tools/dashboard/app: app index.yaml

This is equivalent to the previously-reviewed change to Dmitry's
performance dashboard repo clone:
https://golang.org/cl/35490043

R=golang-dev, dave
CC=golang-dev
https://golang.org/cl/39840043

dashboard/app/index.yaml

@@ -0,0 +1,13 @@
+indexes:
+
+- kind: Commit
+  ancestor: yes
+  properties:
+  - name: Num
+    direction: desc
+
+- kind: Commit
+  ancestor: yes
+  properties:
+  - name: Time
+    direction: desc