nixguix: Document lister

Related to T3781

swh/lister/nixguix/__init__.py

@@ -2,6 +2,32 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+"""
+NixGuix lister
+==============
+
+Nix and Guix package managers are among other things (lazy) functional package managers.
+We cannot easily parse their source declarations as it would require some involved
+computations.
+
+After some discussion and work with both communities, they now expose public manifests
+that the lister consumes to extract origins. Be it the `Guix manifest`_ or the `Nixpkgs
+manifests`_.
+
+4 kinds of origins are listed:
+
+- main `Guix repository`_ or `Nixpkgs repository`_ which are 'git' repositories
+- VCS origins ('git', 'svn', 'hg')
+- unique file ('content')
+- unique tarball ('directory')
+
+.. _Guix repository: https://git.savannah.gnu.org/cgit/guix.git/
+.. _Nixpkgs repository: https://github.com/NixOS/nixpkgs
+.. _Guix manifest: https://guix.gnu.org/sources.json
+.. _Nixpkgs manifests: https://nix-community.github.io/nixpkgs-swh/sources-unstable-full.json
+
+"""
+
 
 def register():
     from .lister import NixGuixLister