md文件是github改良了markdown的语法，用来显示在项目首页的文件。在官方的网址说的很清楚：
GitHub uses what we're calling "GitHub Flavored Markdown" (GFM) for messages, issues, and comments. It differs from standard Markdown (SM) in a few significant ways and adds some additional functionality.
https://help.github.com/articles/github-flavored-markdown
makedown的语法: http://daringfireball.net/projects/markdown/syntax
在线实验: http://daringfireball.net/projects/markdown/dingus
github风格的markdown定义: http://github.github.com/github-flavored-markdown/
标记语言还是蛮多的...这里列一下:
* html系列, 很明显 (html, xhtml, html5)
* xml及docbook
* tex, latex (我感觉pdf本质也算吧??? 加入了二进制似乎)
* 杂/小型的: textile, 这里的md, wiki语法, 甚至bbcode也可以说是.
如何写github md文件 http://www.cnblogs.com/catcherx/archive/2013/02/14/2911166.html
另参考markdown资源：http://www.cnblogs.com/youxin/archive/2012/01/04/2312515.html