This document describes how to configure Xorg XKB from a user's point of view. It covers basic configuration syntax and gives also a few examples.
See also the "X Keyboard Configuration Database" https://www.freedesktop.org/wiki/Software/XKeyboardConfig and
"Custom Keyboard in Linux/X11" http://people.uleth.ca/~daniel.odonnell/Blog/custom-keyboard-in-linuxx11 on Daniel Paul O'Donnell's blog.