Update .gitlab-ci.yml with comments from the article

.gitlab-ci.yml

@@ -1,20 +1,29 @@
+# The only image where dind works on GitLab.com
 image: gitlab/dind
 
+# So we can use Docker inside build script
 services:
 - docker:dind
 
+# Task for GitHub Pages
 pages:
   stage: deploy
+  # cache composer data (especially useful if you set up Satis to download packages)
   cache:
     paths:
     - composer
   script:
   - if [ ! -d composer ]; then mkdir composer; fi
+  # run satis from docker image
   - docker run --rm -i -v `pwd`:/build -v `pwd`/composer:/composer -e COMPOSER_AUTH="$COMPOSER_AUTH" composer/satis
-  - mv output public # gitlab requires directory to be named public
+  # gitlab requires directory to be named public for whatever reason
+  - mv output public
+  # artifacts for Pages
   artifacts:
     paths:
     - public
+    # I don't like any garbage stored in the ci
     expire_in: 1w
+  # do this only on master branch
   only:
   -  master