README.md: add dependabot bagde.

See https://dependabot.com

No functional changes.

[ci skip]

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)
 [![Coverage Status](https://codecov.io/gh/php-coder/mystamps/branch/master/graph/badge.svg)](https://codecov.io/gh/php-coder/mystamps)
 [![PDD Status](http://www.0pdd.com/svg?name=php-coder/mystamps)](http://www.0pdd.com/p?name=php-coder/mystamps)
+[![Dependabot Status](https://api.dependabot.com/badges/status?host=github&repo=php-coder/mystamps)](https://dependabot.com)
 
 ## What's it?