add a release guide

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

.gitlab-ci.yml

@@ -12,5 +12,5 @@ test:
 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 [[ $AUTHORS = *"missing from authors"* ]]; then; echo $AUTHORS && exit 1; fi
+    - if [[ $AUTHORS = *"missing from authors"* ]]; then; { echo $AUTHORS; exit 1; }; fi
   allow_failure: true