feat: check unused package (#232)

* feat: check unused package

update edganiukov/fcm to appleboy/go-fcm

* update readme

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

* update comment

Signed-off-by: Bo-Yi Wu <appleboy.tw@gmail.com>

vendor/github.com/appleboy/go-fcm/README.md

@@ -0,0 +1,64 @@
+# go-fcm
+
+[![GoDoc](https://godoc.org/github.com/edganiukov/fcm?status.svg)](https://godoc.org/github.com/edganiukov/fcm)
+[![Build Status](https://travis-ci.org/edganiukov/fcm.svg?branch=master)](https://travis-ci.org/edganiukov/fcm)
+[![Go Report Card](https://goreportcard.com/badge/github.com/edganiukov/fcm)](https://goreportcard.com/report/github.com/edganiukov/fcm)
+
+This project was forked from [github.com/edganiukov/fcmfcm](https://github.com/edganiukov/fcm).
+
+Golang client library for Firebase Cloud Messaging. Implemented only [HTTP client](https://firebase.google.com/docs/cloud-messaging/http-server-ref#downstream).
+
+More information on [Firebase Cloud Messaging](https://firebase.google.com/docs/cloud-messaging/)
+
+## Getting Started
+
+To install fcm, use `go get`:
+
+```bash
+go get github.com/appleboy/go-fcm
+```
+
+or `govendor`:
+
+```bash
+govendor fetch github.com/appleboy/go-fcm
+```
+
+or other tool for vendoring.
+
+## Sample Usage
+
+Here is a simple example illustrating how to use FCM library:
+
+```go
+package main
+
+import (
+	"github.com/edganiukov/fcm"
+)
+
+func main() {
+	// Create the message to be sent.
+	msg := &fcm.Message{
+     		Token: "sample_device_token",
+      		Data: map[string]interface{}{
+         		"foo": "bar",
+      		}
+  	}
+
+	// Create a FCM client to send the message.
+	client := fcm.NewClient("sample_api_key")
+
+	// Send the message and receive the response without retries.
+	response, err := client.Send(msg)
+	if err != nil {
+		/* ... */
+	}
+	/* ... */
+}
+```
+
+
+#### TODO:
+---------
+- [ ] Retry only failed messages while multicast messaging.