Fix readme to use GitHub Markdown for syntax highlighting

Fix `readme.md` to use [GitHub Flavored Markdown for syntax highlighting](https://help.github.com/articles/github-flavored-markdown#syntax-highlighting) of the code example section.
This looks much nicer on the default project page.

This patch does not change the example code at all, it just removes the leading 4 spaces indentation from every line thus causing noise in the diff.