Update README.md

Author: Ekman

README.md

@@ -50,6 +50,10 @@ $checksum = $luhn->calcChecksum($number);
 $checkDigit = $luhn->calcCheckDigit($number);
 ```
 
+## Versioning
+
+This project complies with [Semantic Versioning](https://semver.org/).
+
 ## Changelog
 
 For a complete list of changes, and how to migrate between major versions, see [releases page](https://github.com/Ekman/luhn-algorithm/releases).