DOC: add in coverage to README.md

Author: jreback

README.md

@@ -36,6 +36,10 @@
     </a>
   </td>
 </tr>
+<tr>
+  <td>Coverage</td>
+  <td><img src="https://codecov.io/github/pydata/pandas/coverage.svg?branch=master" alt="coverage" /></td>
+</tr>
 <tr>
   <td>Conda</td>
   <td>