markdown语法： [to_be_link](#id_name) 标题： ## 2.aaa <a name="id_name"></a>
参考：
http://stackoverflow.com/questions/3292903/in-markdown-what-is-the-best-way-to-link-to-a-fragment-of-a-page-i-e-some-id
原文：http://www.cnblogs.com/bugchecker/p/gitlab_markdown_inner_link.html