`iebaltab` - add this guide for using return matrix to create custom tex tables

Since `v7.0`, `iebaltab` returns all calculated stats in a matrix to allow users to build their own LaTeX tables according to their preferences. I helped a user by typing in up this guide for how to use the return matrix: https://gist.github.com/kbjarkefur/9de3b335cf0e8e40b8ff35087410ebe5

It should be linked to in the help file for `iebaltab` .