maint: Support `guix shell' in Guix's git archive with manifest.scm.

* manifest.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.
* doc/contributing.texi (Building from Git): Mention using it.

Makefile.am

@@ -692,6 +692,7 @@ EXTRA_DIST +=						\
   ROADMAP						\
   TODO							\
   bootstrap						\
+  manifest.scm						\
   build-aux/build-self.scm				\
   build-aux/check-channel-news.scm			\
   build-aux/check-final-inputs-self-contained.scm	\

doc/contributing.texi

@@ -77,6 +77,12 @@ hack on Guix:
 guix shell -D guix --pure
 @end example
 
+or even, from within a Git worktree for Guix:
+
+@example
+guix shell --pure
+@end example
+
 @xref{Invoking guix shell}, for more information on that command.
 
 If you are unable to use Guix when building Guix from a checkout, the

manifest.scm

@@ -0,0 +1,30 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2022 Janneke Nieuwenhuizen <janneke@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+;; GNU Guix development manifest.  To create development environment, run
+;;
+;;     guix shell
+;;
+;; or something like
+;;
+;;     guix shell --pure git git:send-email openssh
+
+(use-modules (guix profiles)
+             (gnu packages package-management))
+
+(package->development-manifest guix)