build: deb repo update test usage instructions.

scripts/build/update_repo/test-update-deb-repo.sh

@@ -1,5 +1,22 @@
 #!/usr/bin/env bash
 
+# Usage
+# docker run -ti --rm -u 0:0 grafana/grafana-ci-deploy:1.1.0 bash
+# in the container:
+# mkdir -p /go/src/github.com/grafana/dist
+# cd /go/src/github.com/grafana
+#
+# outside of container:
+# cd <grafana project dir>/..
+# docker cp grafana <container_name>:/go/src/github.com/grafana/.
+# docker cp <gpg.key used for signing> <container_name>:/private.key
+#
+# in container:
+# gpg --batch --allow-secret-key-import --import /private.key
+# cd dist && wget https://dl.grafana.com/oss/release/grafana_5.4.3_amd64.deb && cd ..
+# run this script:
+# ./script/build/update_repo/test-update-deb-repo.sh <gpg key password>
+
 GPG_PASS=${1:-}
 
 ./scripts/build/update_repo/update-deb.sh "oss" "$GPG_PASS" "v5.4.3" "dist" "grafana-testing-aptly-db" "grafana-testing-repo"