Add CircleCI badge; add table for build badges

+ Remove Go Report Card badge, because it clutters the beginning of the README

README.md

@@ -1,7 +1,11 @@
 serve
 =====
 
-[![Build Status](https://travis-ci.org/philippgille/serve.svg?branch=master)](https://travis-ci.org/philippgille/serve/branches) [![Build status](https://ci.appveyor.com/api/projects/status/nt16vsv7j1yk9yo2/branch/master?svg=true)](https://ci.appveyor.com/project/philippgille/serve/branch/master) [![Go Report Card](https://goreportcard.com/badge/github.com/philippgille/serve)](https://goreportcard.com/report/github.com/philippgille/serve) [![GitHub Releases](https://img.shields.io/github/release/philippgille/serve.svg)](https://github.com/philippgille/serve/releases)
+Windows | macOS | Linux
+--------|-------|------
+[![Build status](https://ci.appveyor.com/api/projects/status/nt16vsv7j1yk9yo2/branch/master?svg=true)](https://ci.appveyor.com/project/philippgille/serve/branch/master) | [![Build Status](https://travis-ci.org/philippgille/serve.svg?branch=master)](https://travis-ci.org/philippgille/serve/branches) | [![CircleCI](https://circleci.com/gh/philippgille/serve.svg?style=svg)](https://circleci.com/gh/philippgille/serve)
+
+[![GitHub Releases](https://img.shields.io/github/release/philippgille/serve.svg)](https://github.com/philippgille/serve/releases)
 
 `serve` starts a simple temporary static file server in your current directory and prints your IP address to share with colleagues.