add codecov coverage badge

Author: funbringer

README.md

@@ -1,5 +1,6 @@
 [![Build Status](https://travis-ci.org/postgrespro/pg_pathman.svg?branch=master)](https://travis-ci.org/postgrespro/pg_pathman)
 [![PGXN version](https://badge.fury.io/pg/pg_pathman.svg)](https://badge.fury.io/pg/pg_pathman)
+[![codecov](https://codecov.io/gh/postgrespro/pg_pathman/branch/master/graph/badge.svg)](https://codecov.io/gh/postgrespro/pg_pathman)
 
 # pg_pathman