Added example for using image digest in the docker run command

Signed-off-by: Shoubhik Bose <sbose78@gmail.com>

Added explanation for the example with image's digest ( as per @thaJeztah 's comment

Signed-off-by: Shoubhik Bose <sbose78@gmail.com>

Wrapped to ~80 chars

Signed-off-by: Shoubhik Bose <sbose78@gmail.com>

docs/reference/run.md

@@ -190,6 +190,11 @@ Images using the v2 or later image format have a content-addressable identifier
 called a digest. As long as the input used to generate the image is unchanged,
 the digest value is predictable and referenceable.
 
+The following example runs a container from the `alpine` image with the 
+`sha256:9cacb71397b640eca97488cf08582ae4e4068513101088e9f96c9814bfda95e0` digest:
+
+    $ docker run alpine@sha256:9cacb71397b640eca97488cf08582ae4e4068513101088e9f96c9814bfda95e0 date
+
 ## PID settings (--pid)
 
     --pid=""  : Set the PID (Process) Namespace mode for the container,