argocd/apps: Add PostgreSQL

The *postgresql* application monitors the PostgreSQL operator and its
assocaited resources.

argocd/applications/postgresql.yaml

@@ -0,0 +1,13 @@
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: postgresql
+  namespace: argocd
+spec:
+  destination:
+    server: https://kubernetes.default.svc
+  project: default
+  source:
+    path: postgresql
+    repoURL: https://git.pyrocufflink.blue/infra/kubernetes.git
+    targetRevision: master