Helm: add NOTES.txt

[krajorama]

What this PR does

Add a NOTES.txt that prints the endpoints that will be usable by the user
for interacting with mimir/gem.

Currently helm does not support rendering the NOTES for testing.
Rage/vote here: helm/helm#6901

Which issue(s) this PR fixes or relates to

N/A

Checklist

• [N/A] Tests updated - not possible at the moment
• [N/A] Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[krajorama]

Rendered output for my gem install:

$ helm -n dev upgrade krajogem ~/go/github.com/grafana/mimir/operations/helm/charts/mimir-distributed -f gem.yaml
[cut]
NOTES:
Welcome to Grafana Enterprise Metrics!

Remote write endpoints for Prometheus or Grafana Agent:
From outside the cluster via ingress:
  http://enterprise-metrics.krajo/api/v1/push

From inside the cluster:
  http://krajogem-enterprise-metrics-gateway.dev.svc:80/api/v1/push

Read address, Grafana data source (Prometheus) URL:
From outside the cluster via ingress:
  http://enterprise-metrics.krajo/prometheus

From inside the cluster:
  http://krajogem-enterprise-metrics-gateway.dev.svc:80/prometheus

[jdbaldry]

Nice!

[krajorama]

From the CI test:

2022-06-23T08:58:48.4538001Z Welcome to Grafana Mimir!
2022-06-23T08:58:48.4538225Z 
2022-06-23T08:58:48.4538446Z Remote write endpoints for Prometheus or Grafana Agent:
2022-06-23T08:58:48.4538831Z Ingress is not enabled, see nginx.ingress values.
2022-06-23T08:58:48.4539173Z From inside the cluster:
2022-06-23T08:58:48.4539800Z   http://mimir-distributed-mvlsf1jk2z-nginx.mimir-distributed-mvlsf1jk2z.svc:80/api/v1/push
2022-06-23T08:58:48.4540226Z 
2022-06-23T08:58:48.4540558Z Read address, Grafana data source (Prometheus) URL:
2022-06-23T08:58:48.4540951Z Ingress is not enabled, see nginx.ingress values.
2022-06-23T08:58:48.4541291Z From inside the cluster:
2022-06-23T08:58:48.4596656Z   http://mimir-distributed-mvlsf1jk2z-nginx.mimir-distributed-mvlsf1jk2z.svc:80/prometheus

[osg-grafana]

Small nit, add "the" in from of "values".