update Forum link to badge style
# iotex-core
-[![Join the forum at https://community.iotex.io/c/research-development/protocol](https://community.iotex.io/uploads/default/original/1X/2e00d16fe3e91748b7cb50d7dd6f0895284bed99.png)](https://community.iotex.io/c/research-development/protocol)
+[![Join the forum](https://img.shields.io/badge/Discuss-IoTeX%20Community-blue)](https://community.iotex.io/c/research-development/protocol)
[![Go version](https://img.shields.io/badge/go-1.11.5-blue.svg)](https://github.com/moovweb/gvm)
[![CircleCI](https://circleci.com/gh/iotexproject/iotex-core.svg?style=svg&circle-token=fe0817d127f251a34b8bdd3336a808c7537e5ec0)](https://circleci.com/gh/iotexproject/iotex-core)
[![Go Report Card](https://goreportcard.com/badge/github.com/iotexproject/iotex-core)](https://goreportcard.com/report/github.com/iotexproject/iotex-core)
