add a release guide

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

.gitlab-ci.yml

@@ -11,7 +11,7 @@ test:
 
 authors check:
   script:
-    - AUTHORS=$(git shortlog -s | sed -e "s/^[0-9\t ]*//"| xargs -i sh -c 'grep -q "{}" AUTHORS.md || echo "{} missing from authors"')
+    - AUTHORS=$(git shortlog -s HEAD | sed -e "s/^[0-9\t ]*//"| xargs -i sh -c 'grep -q "{}" AUTHORS.md || echo "{} missing from authors"')
     - echo $AUTHORS
-    - if [ ! -z $AUTHORS ]; then { echo $AUTHORS; exit 1; }; fi
+    - if [ ! -z "$AUTHORS" ]; then { echo $AUTHORS; exit 1; }; fi
   allow_failure: true