* HACKING (Contributing): New section.
(Building from Git, The Perfect Setup, Coding Style, Submitting Patches):
Move to ...
* doc/guix.texi (Running Guix Before It Is Installed): Likewise.
* doc/contributing.texi: ... here. New file.
* doc.am (EXTRA_DIST): Use it.
* README (Installation): Adapt to it.
* configure.ac (DOT): Likewise.
