docs: Added note on encryption key length (#3995)

README.md

 
 The second set of digits is the encryption key. The Excalidraw server doesn’t know about it. This is what all the participants use to encrypt/decrypt the messages.
 
+> Note: Please ensure that the encryption key is 22 characters long.
+
 ## Shape libraries
 
 Find a growing list of libraries containing assets for your drawings at [libraries.excalidraw.com](https://libraries.excalidraw.com).