Document Debian packaging workflow

Generic parts may be better suited in a proper documentation, and also
help providing guidelines for proper CI/CD.

debian/README.source

@@ -1,2 +1,39 @@
-# In order to import the last upstream $version, run
+## New upstream version
+#
+# Once the package has been tagged (using server/$version format), the
+# last upstream version can be imported using the following command.
+#
+# Note: the git branches master, upstream/latest, and the current debian
+# one have to be checked out first.
+
 gbp import-orig --uscan
+
+## Update Debian changelog
+#
+# One can then edit the Debian changelog with the proper version, e.g.
+# using the following command.
+
+gbp dch -R --multimaint-merge --commit
+
+## Build
+#
+# Then, the package can be build using e.g. the following command.
+#
+# The resulting package should then be available in /tmp/bkctld.
+
+gbp buildpackage
+
+## Lint and checks
+#
+# Lintian may help verify that nothing bad got introduced.
+
+lintian -EI --pedantic /tmp/bkctld/bkctld_*_amd64.changes
+
+# debdiff(1) is useful to check that no files has been removed or added
+# by mistake.
+
+debdiff /tmp/bkctld_{$oldVersion,$newVersion}_all.deb
+
+# diffoscope(1) can go further and show the exact differences.
+
+diffoscope --new-file /tmp/bkctld_{$oldVersion,$newVersion}_all.deb