Update README

README.md

@@ -1,5 +1,5 @@
 haveibeenpwned-library
-============
+======================
 
 [![Build Status](https://travis-ci.org/webeweb/haveibeenpwned-library.svg?branch=master)](https://travis-ci.org/webeweb/haveibeenpwned-library) [![Coverage Status](https://coveralls.io/repos/github/webeweb/haveibeenpwned-library/badge.svg?branch=master)](https://coveralls.io/github/webeweb/haveibeenpwned-library?branch=master) [![Scrutinizer Code Quality](https://scrutinizer-ci.com/g/webeweb/haveibeenpwned-library/badges/quality-score.png?b=master)](https://scrutinizer-ci.com/g/webeweb/haveibeenpwned-library/?branch=master) [![Latest Stable Version](https://poser.pugx.org/webeweb/haveibeenpwned-library/v/stable)](https://packagist.org/packages/webeweb/haveibeenpwned-library) [![Latest Unstable Version](https://poser.pugx.org/webeweb/haveibeenpwned-library/v/unstable)](https://packagist.org/packages/webeweb/haveibeenpwned-library) [![License](https://poser.pugx.org/webeweb/haveibeenpwned-library/license)](https://packagist.org/packages/webeweb/haveibeenpwned-library) [![composer.lock](https://poser.pugx.org/webeweb/haveibeenpwned-library/composerlock)](https://packagist.org/packages/webeweb/haveibeenpwned-library)
 
@@ -9,6 +9,10 @@ projects.
 `haveibeenpwned-library` uses a rolling release based on git master branch which is
 considered stable.
 
+> IMPORTANT NOTICE: This package is still under development. Any changes will be
+> done without prior notice to consumers of this package. Of course this code
+> will become stable at a certain point, but for now, use at your own risk.
+
 ---
 
 ## Compatibility