add codecov badge

Author: YieldRay

README.md

@@ -1,6 +1,7 @@
 # json-rpc-ts
 
 [![deno.land/x](https://shield.deno.dev/x/json_rpc_ts)](https://deno.land/x/json_rpc_ts)
+[![codecov](https://codecov.io/gh/YieldRay/json-rpc-ts/graph/badge.svg?token=BabjRkI8jk)](https://codecov.io/gh/YieldRay/json-rpc-ts)
 [![ci](https://github.com/yieldray/json-rpc-ts/actions/workflows/ci.yml/badge.svg)](https://github.com/yieldray/json-rpc-ts/actions/workflows/ci.yml)
 
 A strictly typed json-rpc(2.0) implementation, zero dependency, minimal abstraction, with simple api