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>

#!/bin/sh

SKIP_ME="### CT_NG SKIP ME ###"

skip_me=$(($(cat "${0}" |@@grep@@ --binary-files=text -o -b -E "^${SKIP_ME}$" |cut -d : -f 1)+${#SKIP_ME}+1))

dd if="${0}" bs=${skip_me} skip=1 2>/dev/null |bzcat

exit 0

### CT_NG SKIP ME ###