Add info on return port.

[adyanth]

Fixes #31

With #pragma HLS INTERFACE mode=s_axilite port=return,

We no longer see the ap_ctrl unconnected.

[adyanth]

Cc: @rck289 @zhenghuama

[zhenghuama]

Hi Adyanth, thanks for the pull request. I double checked your request and in fact, you can set the return port in GUI. The screen shot at the end of 1.3 also showed what the directive tab should look like after we configure everything correctly (with commands for all 3 ports).
However, I apologize that my wordings in the original guide may be unclear and many ppl don't know they must also configure the return port by right-click the module itself (screenshot is too small, easy to be ignored). Also, thank you for providing an alternative (and better) way of configuring via pragma commands.
So, instead of simply merging the request, I updated that section with more detailed explanations.
Thank you!