add note to readme about project being completed (#3724)

README.md

@@ -3,6 +3,10 @@
   <img width="auto" height="50px" src="https://github.com/LAION-AI/Open-Assistant/blob/main/assets/logo_crop.png"/>
 </h1>
 
+<blockquote>
+<p>:memo: <strong>NOTE</strong>: OpenAssistant is completed, and the project is now finished. Thank you to everyone who contributed! Check out our <a href="https://projects.laion.ai/Open-Assistant/blog/2023/10/25/open-assistant-is-completed">blog post</a> for more information.</p>
+</blockquote>
+
 <div align="center">
 
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