kube-prometheus: Add docs on how to set Alertmanager configuration

examples/alertmanager-config-external.jsonnet

@@ -0,0 +1,7 @@
+((import 'kube-prometheus/kube-prometheus.libsonnet') + {
+   _config+:: {
+     alertmanager+: {
+       config: importstr 'alertmanager-config.yaml',
+     },
+   },
+ }).alertmanager.secret