add yaml submodule

README.md

@@ -11,3 +11,4 @@ What's inside
   * [go-mode.el](https://github.com/dominikh/go-mode.el)
   * [linum-plus](https://github.com/emacsmirror/linum-plus)
   * [markdown-mode](https://github.com/emacsmirror/markdown-mode)
+  * [yaml-mode](https://github.com/yoshiki/yaml-mode)