chore(README.md): add a badge for UptimeRobot.

Fix #1152

[skip ci]

Author: php-coder

README.md

@@ -3,6 +3,7 @@
 [![Build Status](https://travis-ci.org/php-coder/mystamps.svg?branch=master)](https://travis-ci.org/php-coder/mystamps)
 [![PDD Status](http://www.0pdd.com/svg?name=php-coder/mystamps)](http://www.0pdd.com/p?name=php-coder/mystamps)
 [![Quality Gate Status](https://sonarcloud.io/api/project_badges/measure?project=ru.mystamps%3Amystamps&metric=alert_status)](https://sonarcloud.io/dashboard?id=ru.mystamps%3Amystamps)
+[![Uptime Statistic](https://badgen.net/uptime-robot/month/ur243278-551fbb732949dbdee27c7552)](https://stats.uptimerobot.com/1jXAjFpgP)
 
 ## What's it?