Migrate android push notification from gcm to fcm

Refs SkygearIO/skygear-SDK-Android#254.

.env.example

@@ -48,6 +48,31 @@ CUSTOM_TOKEN_SECRET="ssosecret"
 # APNS_TOKEN_KEY_PATH=
 ###
 
+###
+# Android Push notification
+# enable fcm for android push notification
+# FCM_ENABLE=true
+# support `server_key` or `service_account` for FCM type
+# server_key - use server key with legacy FCM server API https://firebase.google.com/docs/cloud-messaging/http-server-ref
+# service_account - use service_account with Firebase Cloud Messaging Service https://firebase.google.com/docs/cloud-messaging/admin/send-messages
+# FCM_TYPE=service_account
+
+# for FCM_TYPE=server_key
+# fcm server key
+# FCM_APIKEY=
+
+# for FCM_TYPE=service_account
+# google service account json
+# FCM_SERVICE_ACCOUNT_KEY=
+# google service account json file path
+# FCM_SERVICE_ACCOUNT_KEY_PATH=
+###
+
+# enable baidu push
+# BAIDU_ENABLE=false
+# BAIDU_API_KEY=
+# BAIDU_SECRET_KEY=
+
 ###
 # Skygear supports using Amazon S3 as the default storage backend. Set the AWS
 # access key, secret key, region and bucket by the following environment variables.
@@ -89,11 +114,6 @@ CUSTOM_TOKEN_SECRET="ssosecret"
 # google cloud messaging api key
 # GCM_APIKEY=
 
-# enable baidu push
-# BAIDU_ENABLE=false
-# BAIDU_API_KEY=
-# BAIDU_SECRET_KEY=
-
 # host to listen on. defaults to empty string
 # HOST=localhost
 # defaults to 3000