fix: Add project disclaimer to README (#58)

Author: zharinov

README.md

@@ -21,6 +21,12 @@
   <a href="LICENSE.md"><img src="https://img.shields.io/badge/license-MIT-blue.svg" alt="MIT License"></a>
 </p>
 
+<br/>
+
+<p align="center">
+  <i><strong>⚠️ DISCLAIMER: THE PROJECT IS IN <a href="https://github.com/plotnik-lang/plotnik/edit/master/README.md#roadmap">EARLY STAGE</a> ⚠️</strong></i><br/>
+</p>
+
 <br/>
 <br/>