README instructions are incomplete

[amckinley]

Following the current README and using cortex-manifests.jsonnet.example, I end up with:

(⎈ |development:default) mbp:cortex-jsonnet austin.mckinley$ tk show environments/default
evaluating jsonnet: RUNTIME ERROR: must specify test exporter start time
	/Users/austin.mckinley/robinhood/cortex-jsonnet/vendor/cortex/config.libsonnet:29:31-76	object <anonymous>
	/Users/austin.mckinley/robinhood/cortex-jsonnet/vendor/cortex/test-exporter.libsonnet:8:25-59	object <anonymous>
	/Users/austin.mckinley/robinhood/cortex-jsonnet/vendor/ksonnet-util/kausal.libsonnet:209:10-18	function <anonymous>
		builtin function <flatMap>
	/Users/austin.mckinley/robinhood/cortex-jsonnet/vendor/cortex/test-exporter.libsonnet:20:31-70	thunk from <object <anonymous>>
	/Users/austin.mckinley/robinhood/cortex-jsonnet/vendor/ksonnet.beta.4/k8s.libsonnet:22406:52-56	thunk from <function <anonymous>>
	/Users/austin.mckinley/robinhood/cortex-jsonnet/vendor/ksonnet.beta.4/k8s.libsonnet:22406:43-57	function <anonymous>
	/Users/austin.mckinley/robinhood/cortex-jsonnet/vendor/cortex/test-exporter.libsonnet:20:7-71	object <anonymous>
	/Users/austin.mckinley/robinhood/cortex-jsonnet/vendor/cortex/test-exporter.libsonnet:32:9-34	thunk from <thunk from <object <anonymous>>>
	During manifestation

[amckinley]

Bypassed this by adding test_exporter_enabled: false, to my cortex-manifests.jsonnet.

[tomwilkie]

Thanks Austin! Would you mind opening a PR changing that in https://github.com/grafana/cortex-jsonnet/blob/master/cortex/config.libsonnet#L28?