Add Health Check Documentation

doc/user/admin_area/monitoring/health_check.md

 # Health Check
 
-> [Introduced][ce-3888] in GitLab 8.8.
-
-GitLab provides a health check endpoint for uptime monitoring on the `health_check` web
-endpoint. The health check reports on the overall system status based on the status of
-the database connection, the state of the database migrations, and the ability to write
-and access the cache. This endpoint can be provided to uptime monitoring services like
-[Pingdom][pingdom], [Nagios][nagios-health], and [NewRelic][newrelic-health].
+>**Notes:**
+  - Liveness and readiness probes were [introduced][ce-10416] in GitLab 9.1.
+  - The `health_check` endpoint was [introduced][ce-3888] in GitLab 8.8 and will
+    be deprecated in GitLab 9.1. Read more in the [old behavior](#old-behavior)
+    section.
+
+GitLab provides liveness and readiness probes to indicate service health and
+reachability to required services. These probes report on the status of the
+database connection, Redis connection, and access to the filesystem. These
+endpoints [can be provided to schedulers like Kubernetes][kubernetes] to hold
+traffic until the system is ready or restart the container as needed.
 
 ## Access Token
 
-An access token needs to be provided while accessing the health check endpoint. The current
-accepted token can be found on the `admin/health_check` page of your GitLab instance.
+An access token needs to be provided while accessing the probe endpoints. The current
+accepted token can be found under the **Admin area ➔ Monitoring ➔ Health check**
+(`admin/health_check`) page of your GitLab instance.
 
 ![access token](img/health_check_token.png)
 
 The access token can be passed as a URL parameter:
 
 ```
-https://gitlab.example.com/health_check.json?token=ACCESS_TOKEN
+https://gitlab.example.com/-/readiness?token=ACCESS_TOKEN
 ```
 
-or as an HTTP header:
+which will then provide a report of system health in JSON format:
 
-```bash
-curl --header "TOKEN: ACCESS_TOKEN" https://gitlab.example.com/health_check.json
+```
+{
+  "db_check": {
+    "status": "ok"
+  },
+  "redis_check": {
+    "status": "ok"
+  },
+  "fs_shards_check": {
+    "status": "ok",
+    "labels": {
+      "shard": "default"
+    }
+  }
+}
 ```
 
 ## Using the Endpoint
 
-Once you have the access token, health information can be retrieved as plain text, JSON,
-or XML using the `health_check` endpoint:
+Once you have the access token, the probes can be accessed:
+
+- `https://gitlab.example.com/-/readiness?token=ACCESS_TOKEN`
+- `https://gitlab.example.com/-/liveness?token=ACCESS_TOKEN`
+
+## Status
+
+On failure, the endpoint will return a `500` HTTP status code. On success, the endpoint
+will return a valid successful HTTP status code, and a `success` message.
+
+## Old behavior
+
+>**Notes:**
+  - Liveness and readiness probes were [introduced][ce-10416] in GitLab 9.1.
+  - The `health_check` endpoint was [introduced][ce-3888] in GitLab 8.8 and will
+    be deprecated in GitLab 9.1. Read more in the [old behavior](#old-behavior)
+    section.
+
+GitLab provides a health check endpoint for uptime monitoring on the `health_check` web
+endpoint. The health check reports on the overall system status based on the status of
+the database connection, the state of the database migrations, and the ability to write
+and access the cache. This endpoint can be provided to uptime monitoring services like
+[Pingdom][pingdom], [Nagios][nagios-health], and [NewRelic][newrelic-health].
+
+Once you have the [access token](#access-token), health information can be
+retrieved as plain text, JSON, or XML using the `health_check` endpoint:
 
 - `https://gitlab.example.com/health_check?token=ACCESS_TOKEN`
 - `https://gitlab.example.com/health_check.json?token=ACCESS_TOKEN`
@@ -54,13 +96,13 @@ would be like:
 {"healthy":true,"message":"success"}
 ```
 
-## Status
-
 On failure, the endpoint will return a `500` HTTP status code. On success, the endpoint
 will return a valid successful HTTP status code, and a `success` message. Ideally your
 uptime monitoring should look for the success message.
 
+[ce-10416]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3888
 [ce-3888]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3888
 [pingdom]: https://www.pingdom.com
 [nagios-health]: https://nagios-plugins.org/doc/man/check_http.html
 [newrelic-health]: https://docs.newrelic.com/docs/alerts/alert-policies/downtime-alerts/availability-monitoring
+[kubernetes]: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-probes/