add codecov badge

Author: mattn

README.mkd

@@ -1,5 +1,6 @@
 # Vim Script Parsers
 
-[![Build Status](https://travis-ci.org/vim-jp/vim-vimlparser.svg?branch=master)](https://travis-ci.org/vim-jp/vim-vimlparser)
+[![Build Status](https://travis-ci.org/vim-jp/vim-vimlparser.svg?branch=master)](https://travis-ci.org/vim-jp/vim-vimlparser) [![codecov](https://codecov.io/gh/vim-jp/vim-vimlparser/branch/master/graph/badge.svg)](https://codecov.io/gh/vim-jp/vim-vimlparser)
+
 
 TODO: write something.