add README with instructions on how to use this repo

README.md

+# LaTeX CI Template
+
+This project is a template to automatically build a LaTeX project with Gitlab CI (Continuous Integration).
+
+## How to use it
+
+* [Fork](https://gitlab.viarezo.fr/dric0/latex-ci-template/forks/new) this project.
+* Replace the content of `main.tex` with your LaTex code.
+* Edit the [Download your PDF](#download-your-pdf) section : replace every `dric0` occurence (there are 3) with your gitlab username, and edit the repository name if you have changed it.
+* Have fun
+
+## Download your PDF
+
+[![pipeline status](https://gitlab.viarezo.fr/dric0/latex-ci-template/badges/master/pipeline.svg)](https://gitlab.viarezo.fr/dric0/latex-ci-template/commits/master)
+
+[Download](https://gitlab.viarezo.fr/dric0/latex-ci-template/-/jobs/artifacts/master/raw/main.pdf?job=build)