add doc

Docker-DCO-1.1-Signed-off-by: Victor Vieux <vieux@docker.com> (github: vieux)

docs/sources/reference/api/docker_remote_api.md

@@ -36,6 +36,11 @@ You can still call an old version of the api using
 
 ### What's new
 
+`GET /_ping`
+
+**New!**
+You can now ping the server via the `_ping` endpoint.
+
 `GET /events`
 
 **New!**

docs/sources/reference/api/docker_remote_api_v1.11.md

@@ -1162,6 +1162,26 @@ Show the docker version information
     -   **200** – no error
     -   **500** – server error
 
+### Ping the docker server
+
+`GET /_ping`
+
+Ping the docker server
+
+    **Example request**:
+
+        GET /_ping HTTP/1.1
+
+    **Example response**:
+
+        HTTP/1.1 200 OK
+
+        OK
+
+    Status Codes:
+
+    -   **200** - no error
+
 ### Create a new image from a container's changes
 
 `POST /commit`