add codecov badge

Signed-off-by: Matthew Peveler <matt.peveler@gmail.com>

Author: MasterOdin

README.md

@@ -1,6 +1,7 @@
 # predicate-types.js
 
 [![Test](https://github.com/OpenReasoning/predicate-types.js/workflows/Test/badge.svg?branch=master&event=push)](https://github.com/OpenReasoning/predicate-types.js/actions?query=workflow%3ATest+event%3Apush+branch%3Amaster)
+[![codecov](https://codecov.io/gh/OpenReasoning/predicate-types.js/branch/master/graph/badge.svg)](https://codecov.io/gh/OpenReasoning/predicate-types.js)
 ![NPM Version](https://img.shields.io/npm/v/@openreasoning/predicate-types.svg)
 
 ## Install