Add example in the project.

[tzzed]

Hi Guys congrats again.

I suggest to add a package with an example and mentionne it in the README file. something like this.

import (
	"fmt"
	"log"
	"os"

	"github.com/phoops/ngsiv2/client"
)

func main() {
	url := os.Getenv("ORION_BROKER")
	cli, err := client.NewNgsiV2Client(client.SetUrl(url))
	if err != nil {
		log.Fatalln(err)
	}

	e, err := cli.RetrieveEntity("urn:ngsi-ld:Product:001", client.RetrieveEntitySetType("Product"))
	if err != nil {
		log.Fatalln(err)
	}
	
	b, _ := e.MarshalJSON()
	fmt.Println(string(b))
}

[zarelit]

This is a very welcomed suggestion, I think we should have examples of interactions using client and also how to move around entities, set and read attributes, in particular with custom types and geolocation