command-line-interface: Add --pid-file to 'create'

You can get the PID by calling 'state' [1], and container PIDs may not
be portable [2].  But it's a common way for interfacing with init
systems like systemd [3], and for this first pass at the command line
API folks are ok with some Linux-centrism [4].

[1]: https://github.com/opencontainers/runtime-spec/pull/511/files#r70353376
     Subject: Add initial pass at a cmd line spec
[2]: opencontainers#459
     Subject: [ Runtime ] Allow for excluding pid from state
[3]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-07-20-21.03.log.html#l-69
[4]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-07-13-17.03.log.html#l-39

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

command-line-interface.md

@@ -31,6 +31,7 @@ This command SHOULD exit as quickly as possible after creating the container, wi
     * *`<ID>`* Set the container ID to create.
 * *Options*
     * *`--bundle <PATH>`* Override the path to the [bundle directory][bundle] (defaults to the current working directory).
+    * *`--pid-file <PATH>`* The runtime MUST write the container PID to this path.
 * *Standard streams:*
     * *stdin:* The runtime MUST attach its stdin directly to the container process without reading from it.
     * *stdout:* The runtime MUST attach its stdout directly to the container process without writing to it.