Document gbp import-ref usage

debian/README.source

+Starting with 2.0.0, we are merging the “signed” upstream tags as
+upstream, instead of using the tarball that doesn’t ship tests.
+
+# Example to import upstream 2.0.1 version
+version=2.0.1
+# We still use the upstream branch to drop the .gitattributes file from upstream
+gbp import-ref --upstream-tag=%\(version%~%-\)s -u$version --debian-branch=upstream
+gbp import-ref --upstream-tree=BRANCH -u$version

debian/gbp.conf

+[DEFAULT]
+pristine-tar = True
+pristine-tar-commit = True
+
+## Once --filter support gets added to gbp import-ref, we should be able
+## to simplify the workflow and ignore the upstream branch.
+# filter = [ '.gitattributes' ]
+# upstream-tag = %(version%~%-)s