Include gnulib and paxutils as submodules.

* .gitmodules: New file.
* README-alpha: Update.
* README-hacking: Update.
* bootstrap: Install slightly modified version from the gnulib repo.
* bootstrap.conf: Add paxutils-related stuff.
* .gitignore: Update.
* doc/.gitignore: Update.
* po/.gitignore: Update.
* gnu/Makefile.am

README-hacking

@@ -12,7 +12,6 @@ version.
 - Autoconf <http://www.gnu.org/software/autoconf/>
 - M4 <http://www.gnu.org/software/m4/>
 - Texinfo <http://www.gnu.org/software/texinfo>
-- Gnulib <http://www.gnu.org/software/gnulib>
 - Git <http://git.or.cz>
 
 * Bootstrapping
@@ -33,23 +32,14 @@ Once done, proceed as described in the file README (section
 INSTALLATION).
 
 Normally you will have to run bootstrap only once. However, if you
-intend to hack on GNU tar, you might need to run it again later. In
-this case, you will probably want to save some time and bandwidth by
-avoiding downloading the same files again. If so, create in GNU tar
-root directory the file named '.bootstrap' with the following
-contents:
-
-   --gnulib-srcdir=$HOME/gnulib
-
-Replace '$HOME/gnulib' with the actual directory where the Gnulib
-sources reside.
-
-For more information about 'bootstrap', run 'bootstrap --help'.
+intend to hack on GNU tar, you might need to run it again later.
+There are lots of options that you may find useful in this case.
+See 'bootstrap --help' for a detailed list.
 
 
 * Copyright information
 
-Copyright 2007-2009, 2013-2014 Free Software Foundation, Inc.
+Copyright 2007-2009, 2013-2015 Free Software Foundation, Inc.
 
 This file is part of GNU tar.