Add explanatory images to pre-built measures documentation

<a href="https://github.com/lrodriguezlujan"><img src="https://avatars2.githubusercontent.com/u/15161115?v=4" align="left" width="96" height="96" hspace="10"></img></a> **Issue by [lrodriguezlujan](https://github.com/lrodriguezlujan)**
_Tuesday Nov 29, 2016 at 11:07 GMT_
_Originally opened as https://github.com/lrodriguezlujan/neurostr/issues/113_

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