Add support for GNU Guix

* .gitignore: Ignore changes to manifest.scm.
* Makefile: Add manifest.scm by linking it from the admin directory.
* README: Document how to use Guix.

.gitignore

@@ -14,3 +14,4 @@ core
 /archive-devel/
 /admin
 GNUmakefile
+manifest.scm

Makefile

@@ -1,9 +1,12 @@
 
-setup: GNUmakefile admin
+setup: GNUmakefile manifest.scm admin
 
 GNUmakefile: admin
 	ln -s admin/GNUmakefile ./
 
+manifest.scm: admin
+	ln -s admin/elpa-manifest.scm $@
+
 admin:
 	git remote add --no-tags -ft elpa-admin \
 	    gnu-elpa git://git.sv.gnu.org/emacs/elpa.git

README.org

@@ -1,5 +1,5 @@
 #+TITLE: NonGNU ELPA README
-#+DATE: 2021-12-30
+#+DATE: 2022-06-05
 
 Copyright (C) 2020-2022 Free Software Foundation, Inc. \\
 See the end of the file for license conditions.
@@ -88,6 +88,14 @@ to do:
     rm archive/[PKG]-[VERSION].tar; make build/[PKG]
 #+end_src
 
+If you have the [[https://guix.gnu.org/][GNU Guix]] package manager installed, it can take care
+of all the dependencies necessary for the ELPA build system to run.
+You can initialise a shell session by running
+
+#+begin_src sh
+  guix shell -m manifest.scm
+#+end_src
+
 * Using packages in-place
 
 The packages can be installed and used in-place.  First you need to make