README: add a codecov.io badge.

* README.md: Add a codecov.io badge.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>

Author: gvvaughan

README.md

@@ -5,6 +5,7 @@ by the [prototype project][github]
 
 [![License](http://img.shields.io/:license-mit-blue.svg)](http://mit-license.org)
 [![travis-ci status](https://secure.travis-ci.org/lua-stdlib/prototype.png?branch=master)](http://travis-ci.org/lua-stdlib/prototype/builds)
+[![codecov.io](https://codecov.io/github/lua-stdlib/prototype/coverage.svg?branch=master)](https://codecov.io/github/lua-stdlib/prototype?branch=master)
 [![Stories in Ready](https://badge.waffle.io/lua-stdlib/prototype.png?label=ready&title=Ready)](https://waffle.io/lua-stdlib/prototype)