docs: add codecov badge

Author: felixfbecker

README.md

@@ -1,6 +1,7 @@
 # CodeIntellify
 
 [![build](https://badge.buildkite.com/da1855cc6c9b02ddfa1df69599aacecd1317db8f6765edfa8b.svg?branch=master)](https://buildkite.com/sourcegraph/codeintellify)
+[![codecov](https://codecov.io/gh/sourcegraph/codeintellify/branch/master/graph/badge.svg?token=1Xk7sdvG0y)](https://codecov.io/gh/sourcegraph/codeintellify)
 [![code style: prettier](https://img.shields.io/badge/code_style-prettier-ff69b4.svg)](https://github.com/prettier/prettier)
 
 > Adds code intelligence to code views on the web