add a release guide

Even though there are very few steps involved, it is good to have it documented.

.gitlab-ci.yml

@@ -13,4 +13,5 @@ authors:
   script:
     - AUTHORS=`git shortlog -s | sed -e "s/^[0-9\t ]*//"| xargs -i sh -c 'grep -q "{}" AUTHORS.md || echo "{} missing from authors"'`
     - if [ ! -z $AUTHORS ]; then { echo $AUTHORS; exit 1; }; fi
+    - [ ! -z $AUTHORS ] && { echo $AUTHORS; exit 1; } || exit 0
   allow_failure: true