md文件是github改良了markdown的語法，用來顯示在項目首頁的文件。在官方的網址說的很清楚：
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的語法: http://daringfireball.net/projects/markdown/syntax
在線實驗: http://daringfireball.net/projects/markdown/dingus
github風格的markdown定義: http://github.github.com/github-flavored-markdown/
標記語言還是蠻多的...這里列一下:
* html系列, 很明顯 (html, xhtml, html5)
* xml及docbook
* tex, latex (我感覺pdf本質也算吧??? 加入了二進制似乎)
* 雜/小型的: textile, 這里的md, wiki語法, 甚至bbcode也可以說是.
如何寫github md文件 http://www.cnblogs.com/catcherx/archive/2013/02/14/2911166.html
另參考markdown資源：http://www.cnblogs.com/youxin/archive/2012/01/04/2312515.html