Add StyleCI badge to README.md

Author: vladdeSV

README.md

@@ -3,6 +3,7 @@
 # National Identification Numbers
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 [![develop](https://github.com/vladdeSV/php-nin/workflows/develop/badge.svg?branch=develop)](https://github.com/vladdeSV/php-nin/actions?query=workflow%3Adevelop)
 
 This library validates the structure of individual identification numbers. Currently supporting all Nordic countries.