runtime: Add 'creating' to state status

To distinguish between "we're still setting this container up" and
"we're finished setting up; you can call 'start' if you like".

Also reference the lifecycle steps, because you can't be too explicit

Signed-off-by: W. Trevor King <wking@tremily.us>

runtime.md

@@ -15,9 +15,10 @@ This MUST be unique across all containers on this host.
 There is no requirement that it be unique across hosts.
 * **`status`**: (string) is the runtime state of the container.
 The value MAY be one of:
-    * `created`: the container process has neither exited nor executed the user-specified code
-    * `running`: the container process has executed the user-specified code but has not exited
-    * `stopped`: the container process has exited
+    * `creating`: the container is being created (step 2 in the [lifecycle](#lifecycle))
+    * `created`: the runtime has finished the [create operation](#create) (after step 2 in the [lifecycle](#lifecycle)), and the container process has neither exited nor executed the user-specified code
+    * `running`: the container process has executed the user-specified code but has not exited (after step 4 in the [lifecycle](#lifecycle))
+    * `stopped`: the container process has exited (step 5 in the [lifecycle](#lifecycle))
 
   Additional values MAY be defined by the runtime, however, they MUST be used to represent new runtime states not defined above.
 * **`pid`**: (int) is the ID of the container process, as seen by the host.