Update README.md

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@@ -37,7 +37,9 @@ Important: The USERNAME is your phone number in international format and must in
 
         signal-cli -u USERNAME register
         
-  You can register Signal using a land line number. In this case you can skip SMS verification process and jump directly to the voice call verification by adding the --voice switch at the end of above register command.
+  You can register Signal using a land line number. In this case you can skip SMS verification process and jump directly to the voice call verification by adding the `--voice` switch at the end of above register command.
+  
+  Registering may require solving a CAPTCHA challenge: [Registration with captcha](https://github.com/AsamK/signal-cli/wiki/Registration-with-captcha)
 
 * Verify the number using the code received via SMS or voice, optionally add `--pin PIN_CODE` if you've added a pin code to your account