add some new badges to the readme (#268)

This commit adds the following badges

- the project License
- go report card score
- number of Projects go-gitlab is used by (sourced fom sourcegraph)
- number of open Github issues

It also rearranges them slightly.
Signed-off-by: Gavin-John Noonan <mail@gjnoonan.co.uk>

README.md

@@ -2,8 +2,12 @@
 
 A GitLab API client enabling Go programs to interact with GitLab in a simple and uniform way
 
-**Documentation:** [![GoDoc](https://godoc.org/github.com/xanzy/go-gitlab?status.svg)](https://godoc.org/github.com/xanzy/go-gitlab)
-**Build Status:** [![Build Status](https://travis-ci.org/xanzy/go-gitlab.svg?branch=master)](https://travis-ci.org/xanzy/go-gitlab)
+[![Build Status](https://travis-ci.org/xanzy/go-gitlab.svg?branch=master)](https://travis-ci.org/xanzy/go-gitlab)
+[![GitHub license](https://img.shields.io/github/license/xanzy/go-gitlab.svg)](https://github.com/xanzy/go-gitlab/blob/master/LICENSE)
+[![Sourcegraph](https://sourcegraph.com/github.com/xanzy/go-gitlab/-/badge.svg)](https://sourcegraph.com/github.com/xanzy/go-gitlab?badge)
+[![GoDoc](https://godoc.org/github.com/xanzy/go-gitlab?status.svg)](https://godoc.org/github.com/xanzy/go-gitlab)
+[![Go Report Card](https://goreportcard.com/badge/github.com/xanzy/go-gitlab)](https://goreportcard.com/report/github.com/xanzy/go-gitlab)
+[![GitHub issues](https://img.shields.io/github/issues/xanzy/go-gitlab.svg)](https://github.com/xanzy/go-gitlab/issues)
 
 ## NOTE