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:
-    - GITLOG=(echo $(git shortlog -s))
+    - GITLOG=`echo $(git shortlog -s)`
     - echo $GITLOG
     - AUTHORS=$(git shortlog -s | sed -e "s/^[0-9\t ]*//"| xargs -i sh -c 'grep -q "{}" AUTHORS.md || echo "{} missing from authors"')
     - echo $AUTHORS