doc(syntax_highlight_in_latex.md): explain how to use minted and Pygments

doc/syntax_highlight_in_latex.md

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+We provide a way to syntax-highlight Lean code in LaTeX documents. 
+It requires a Python package `Pygments` and a LaTeX package `minted`.
+
+
+Python Package: `Pygments`
+--------------------------
+
+Checkout [Pygments development repository][pygments-dev] and build Pygments:
+
+```bash
+hg clone https://bitbucket.org/birkenfeld/pygments-main/
+cd pygmentsd-main
+make mapfiles
+sudo ./setup.py install
+````
+
+[pygments-dev]: https://bitbucket.org/birkenfeld/pygments-main
+
+
+LaTeX package: `minted`
+-----------------------
+
+Save the latest version of [minted.sty][minted.sty] at the working directory.
+
+[minted]: https://github.com/gpoore/minted
+[minted.sty]: https://raw.githubusercontent.com/gpoore/minted/master/source/minted.sty
+
+
+How to use them (example)
+-------------------------
+
+Please save the following sample LaTeX file as `test.tex`:
+
+```latex
+\documentclass{article}
+\usepackage{minted}
+\setminted{encoding=utf-8}
+\usepackage{fontspec}
+\setmainfont{FreeSerif}
+\setmonofont{FreeMono}
+\usepackage{fullpage}
+\begin{document}
+\begin{minted}[mathescape,
+               linenos,
+               numbersep=5pt,
+               frame=lines,
+               framesep=2mm]{Lean}
+theorem mul_cancel_left_or {a b c : ℤ} (H : a * b = a * c) : a = 0 ∨ b = c :=
+have H2 : a * (b - c) = 0, by simp,
+have H3 : a = 0 ∨ b - c = 0, from mul_eq_zero H2,
+or.imp_or_right H3 (assume H4 : b - c = 0, sub_eq_zero H4)
+\end{minted}
+\end{document}
+```
+
+You can compile `test.tex` by executing the following command:
+
+```bash
+xelatex --shell-escape test
+```
+
+Some remarks:
+
+ - `xelatex` is required to handle unicode in LaTeX. 
+ - `--shell-escape` option is needed to allow `xelatex` to execute `pygmentize` in a shell.
+
+ 
+Contribute
+----------
+
+Please fork [Leonardo de Moura's Pygments repository][pygments-lean], improve it, and make a pull-request.
+
+[pygments-lean]: https://bitbucket.org/leodemoura/pygments-main