Document new optional support for ConfigMap and Secret

[fraenkel]

Corresponds to proposal kubernetes/community#175

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This change is 

[chenopis]

@fraenkel One nitpick: on line 320, can you change that sentence to "When this pod is run, the output will be:" because when you say "will include the lines:" I'm expecting there to be some sort of output. Also, it will match better with the other lines later on where you say that.

[fraenkel]

@chenopis No problem. I was struggling with some of the wording.
I changed it to just say the output will be empty. Looking at empty output is confusing, did the author forget to write something?

[chenopis]

@fraenkel Yeah, exactly. Thx!