readme: hint to unmaintained

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README.md

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 # Signal Android 
 
+**Unmaintained fork of Signal**
+
+
 Signal is a simple, powerful, and secure messenger.
 
 Signal uses your phone's data connection (WiFi/3G/4G/5G) to communicate securely. Millions of people use Signal every day for free and instantaneous communication anywhere in the world. Send and receive high-fidelity messages, participate in HD voice/video calls, and explore a growing set of new features that help you stay connected. Signal’s advanced privacy-preserving technology is always enabled, so you can focus on sharing the moments that matter with the people who matter to you.