feat: update in ci/cd script and readme

.gitlab-ci.yml

@@ -3,7 +3,7 @@ image: git-r3lab.uni.lu:4567/r3/docker/latex:latest
 stages:
   - build
 
-build:pages:
+build:pdf file:
   stage: build
   artifacts:
     expire_in: 1 day

README.md

@@ -12,3 +12,7 @@ edges are cleaned up; in the meantime it is likely better to just follow the
 example layouts in `slides.tex`.
 
 Open issues/send merge requests in case of any ideas.
+
+# How to generate the PDF file using Gitlab CI/CD?
+Go to "CI/CD" tab on the left-hand side. Find the button to start "build:pdf file" task, and click it.
+After a few minutes the task should finish, and you'll find on the right-hand-side an option to download the "Arftifacts" - an archive containing the generated PDF file.