README.ai: package instructions

README.ai

+Mailman Debian packages
+=======================
+
+This repo contains a copy of the mailman2 git repository from Debian
+(https://salsa.debian.org/mailman-team/mailman2.git) and will be built as-is
+(debian/patches will be removed before building the source package) by
+gitlab-ci, thus all commits (ours + debian's) will be included as well.
+
+Upgrade
+=======
+
+When the Debian git repository is updated (e.g. new mailman version) it is
+possible to merge 'master' branch back into this repository:
+
+  $ git remote add salsa https://salsa.debian.org/mailman-team/mailman2.git
+  $ git fetch salsa
+  $ git merge salsa/master
+
+Fix conflicts if any and push, the resulting package should be built as expected.