add instructions to push alpha tag

Author: BenTheElder

hack/release/create.sh

@@ -82,4 +82,5 @@ echo " - git push"
 echo " - File a PR with these pushed commits"
 echo " - Merge the PR"
 echo " - git push ${UPSTREAM} v${1}"
+echo " - git push ${UPSTREAM} v${2}-alpha"
 echo " - Create a GitHub release from the pushed tag v${1}"