Changes made for LaTeX.to to the Metropolis template

Template page: https://latex.to/templates/github/metropolis/
Repository:    https://github.com/matze/mtheme
Commit:        2fa6084b9d34fec9d2d5470eb9a17d0bf712b6c8
Author:        Matthias Vogelgesang
License:       CC BY-SA 4.0, see README.md

Changes from the original (a/) to this copy (b/):

--- a/demo.tex
+++ b/demo.tex
@@ -1,3 +1,4 @@
+% !TeX program = xelatex
 \documentclass[10pt]{beamer}
 
 \usetheme{metropolis}

Why each change was made, in the order of the diff:

- demo.tex: the first line is added. It selects XeLaTeX, the engine the
  repository's Makefile builds the demo with. Auto cannot see the font setup
  inside the Metropolis theme that TeX Live installs, and compiles the
  unchanged demo with pdfLaTeX, in Computer Modern Sans where the theme is
  made for Fira Sans.