DL4J Keras Import: add note on RGB/BGR for CNNs

[AlexDBlack]

One trap for Keras import of CNNs in RGB vs. BGR layout for images.

It's straightforward to configure (see test below) but many users don't even think of this when importing their nets.
We should note this prominently somewhere in the Keras import docs.

https://github.com/deeplearning4j/deeplearning4j/blob/master/datavec/datavec-data/datavec-data-image/src/test/java/org/datavec/image/transform/TestImageTransform.java#L247

[maxpumperla]

good point, thanks