content: Add Installation guide of mdformat

The installation guide is quoted from [1], for full GitHub markdown
features support, additional packages need to be installed.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

Author: RuoqingHe

src/repo-management/firecracker/markdown.md

@@ -16,3 +16,14 @@ It is a good idea to format these files, but I don't prefer doing it without
 `--check` flag. I think it would be better to tell developers that some `.md`
 files are not in a preferred form that `mdformat` thinks, instead of changing it
 directly and developers may well get confuse on what is going to be committed.
+
+## Install mdformat
+
+Installation guide of `mdformat` is documented
+[here](https://github.com/executablebooks/mdformat/blob/master/README.md#installing).
+GibHub's Markdown renderer supports syntax enxtensions not included in the GFM
+specification, so for full GitHub support we should install it with:
+
+```sh
+pip install mdformat-gfm mdformat-frontmatter mdformat-footnote mdformat-gfm-alerts
+```