Make pid option on platforms other than linux

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

runtime.md

@@ -22,7 +22,7 @@ The state of a container includes the following properties:
     * `stopped`: the container process has exited (step 7 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, REQUIRED when `status` is `created` or `running`) is the ID of the container process, as seen by the host.
+* **`pid`** (int, REQUIRED when `status` is `created` or `running` on Linux, OPTIONAL on other platforms) is the ID of the container process, as seen by the host.
 * **`bundle`** (string, REQUIRED) is the absolute path to the container's bundle directory.
     This is provided so that consumers can find the container's configuration and root filesystem on the host.
 * **`annotations`** (map, OPTIONAL) contains the list of annotations associated with the container.