Add alert configuration

```
# alerting:
# backends:
# - name: cloud-monitoring
# class: CloudMonitoring
# projectId: slo-generator-ci-a2b4
# notifications:
# - channel-1
# - channel-2

# - name: prometheus-alert-manager
# class: AlertManager
# url: ${ALERT_MANAGER_URL}

# alerts:
# - name: high-burn-rate
# backend: cloud-monitoring
# displayName: SLO burn rate too high
# combiner: OR
# conditions:
# - displayName: SLO burn rate
# conditionThreshold:
# aggregations:
# alignmentPeriod: 60s
# crossSeriesReducer: REDUCE_MEAN
# groupByFields:
# - project
# - resource.label.instance_id
# - resource.label.zone
# perSeriesAligner: ALIGN_MAX
# comparison: COMPARISON_GT
# duration: 900s
# filter: >
# metric.type=compute.googleapis.com/instance/cpu/utilization
# resource.type=gce_instance
# thresholdValue: auto
# trigger:
# count: 1
```