add coverage badge (#383)

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>
Reviewed-on: #383
Co-authored-by: Vasiliy Tolstov <v.tolstov@unistack.org>
Co-committed-by: Vasiliy Tolstov <v.tolstov@unistack.org>

README.md

@@ -1,4 +1,5 @@
 # Micro [![License](https://img.shields.io/:license-apache-blue.svg)](https://opensource.org/licenses/Apache-2.0) [![Doc](https://img.shields.io/badge/go.dev-reference-007d9c?logo=go&logoColor=white&style=flat-square)](https://pkg.go.dev/github.com/unistack-org/micro/v3?tab=overview) [![Status](https://git.unistack.org/unistack-org/micro/actions/workflows/job_tests.yml/badge.svg?branch=v3)](https://git.unistack.org/unistack-org/micro/actions?query=workflow%3Abuild+branch%3Av3+event%3Apush) [![Lint](https://goreportcard.com/badge/go.unistack.org/micro/v3)](https://goreportcard.com/report/go.unistack.org/micro/v3)
+![Coverage](https://img.shields.io/badge/Coverage-45.1%25-yellow)
 
 Micro is a standard library for microservices.