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

examples/alertmanager-config.jsonnet

@@ -0,0 +1,22 @@
+((import 'kube-prometheus/kube-prometheus.libsonnet') + {
+   _config+:: {
+     alertmanager+: {
+       config: |||
+         global:
+           resolve_timeout: 10m
+         route:
+           group_by: ['job']
+           group_wait: 30s
+           group_interval: 5m
+           repeat_interval: 12h
+           receiver: 'null'
+           routes:
+           - match:
+               alertname: DeadMansSwitch
+             receiver: 'null'
+         receivers:
+         - name: 'null'
+       |||,
+     },
+   },
+ }).alertmanager.secret