Add go get instructions, could be confusing for developers new to go

When running the example with iglo installed at GOPATH, it can be confusing to developers unfamiliar with golang.
```bash
stage:examples khangtoh$ go run api-server.go 
api-server.go:4:2: cannot find package "github.com/subosito/iglo" in any of:
	/usr/local/go/src/pkg/github.com/subosito/iglo (from $GOROOT)
	/Users/khangtoh/go/src/github.com/subosito/iglo (from $GOPATH)
stage:examples khangtoh$ 
```

Add instructions for go get