Add "Extending FreeIPA" developer guide
    
    "Extending FreeIPA" is a developer guide of FreeIPA core framework.
    Please make sure to improve the guide every time parts of the core framework
    are affected by your changes. This document ideally should correspond to
    the current state of the framework.
    
    The Guide is written using Emacs Org Mode but can be edited with any
    plain text editing tool. Emacs is only required to convert it to
    distribution formats like HTML and TXT.
    
    See guide/Makefile for building the guide and README for details of
    the build environment.