Update README - require semantic versioning convention

Author: populov

README.md

@@ -77,8 +77,8 @@ Root of your rule builder.
 2. Develop locally as usual. **Code must follow [PSR-1](http://www.php-fig.org/psr/psr-1/), [PSR-2](http://www.php-fig.org/psr/psr-2/)** -
     run [PHP_CodeSniffer](https://github.com/squizlabs/PHP_CodeSniffer) to ensure, that code follows style guides
 3. **Cover added functionality with unit tests** and run [PHPUnit](https://phpunit.de/) to make sure, that all tests pass
-4. Update [README.md](README.md) to describe new or changed functionality.
-5. Add changes description to [CHANGES.md](CHANGES.md) file.
+4. Update [README.md](README.md) to describe new or changed functionality
+5. Add changes description to [CHANGES.md](CHANGES.md) file. Use [Semantic Versioning](https://semver.org/) convention to determine next version number.
 6. When ready, create pull request
 
 ### Make shortcuts