docs: add an in-depth explanations of the build steps

The build process is quite complex: gcc is built three times, there are
two C library steps, there are those companion libraries...

People often wonder what all these steps do, and why they are needed.

Recently, someone proposed a tutorial on the crossgcc mailing list:
http://sourceware.org/ml/crossgcc/2011-01/msg00059.html

This meant that there was a need for such a tutorial, and explanations
on how a toolchain is built. So i decide to extend my answers:
http://sourceware.org/ml/crossgcc/2011-01/msg00060.html
http://sourceware.org/ml/crossgcc/2011-01/msg00125.html

into proper documentation in crosstool-NG.

Thanks go to Francesco for suggesting this. He has a fine tutorial
for beginners there:
http://fturco.org/wiki/doku.php?id=debian:cross-compiler

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 This is the README for crosstool-NG

 Crosstool-NG follows the autoconf dance. So, to get you

 kick-started, just run:

   ./configure --help

 You will find the documentation in the directory 'docs'.

 Here is a quick overview of what you'll find there:

   0 - Table of content

   1 - Introduction

   2 - Installing crosstool-NG

   3 - Configuring a toolchain

   4 - Building the toolchain

   5 - Using the toolchain

   6 - Toolchain types

   7 - Contributing

   8 - Internals

   A - Credits

   B - Known issues

   C - Misc. tutorials

 You can also point your browser at:

   http://ymorin.is-a-geek.org/projects/crosstool

 Aloha!