[doc] README: CI badge, link to pdf

README.md

+[![pipeline status](https://gitlab.inria.fr/nix-tutorial/presentation-slides/badges/master/pipeline.svg)](https://gitlab.inria.fr/nix-tutorial/presentation-slides/pipelines)
+[![latest pdf](https://img.shields.io/badge/latest%20pdf-online-brightgreen)](https://gitlab.inria.fr/nix-tutorial/presentation-slides/-/jobs/artifacts/master/raw/slides.pdf\?job\=slides)
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 Repository containing the slides introducing the Nix tutorial about reproducible experiments with Nix.
 
 To build the presentation: