add README example

Author: gravityblast

README.md

@@ -1,2 +1,36 @@
 # etherscan-go
 etherscan.io API client in Go
+
+```go
+// main.go
+
+package main
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/gravityblast/etherscan"
+)
+
+func checkErr(err error) {
+	if err != nil {
+		log.Fatal(err)
+	}
+}
+
+func main() {
+	c, err := etherscan.NewClient(etherscan.Mainnet, "YOUR_API_KEY")
+	checkErr(err)
+
+	resp, err := c.Account("0x0000000000000000000000000000000000000000")
+	checkErr(err)
+
+	fmt.Printf("%+v\n", resp)
+}
+```
+
+```bash
+go run main.go
+&{Status:1 Message:OK Result:+7641879211751631214533}
+```