Update README.md

Author: dl9sec

README.md

@@ -3,6 +3,7 @@ Arduino SIP library with UDP communications. Original class and methods authored
 Can be used do do a notifying SIP call through e.g. a FRITZ!Box.
 
 Tested with NodeMCU 1.0 (AI-Thinker ESP8266MOD ESP-12E).
+Because of this, _"architectures"_ in _"library.properties"_ is currently set to _"esp8266"-only_. If you want to use the library on another Arduino architecture, replace the _"esp8266"_ with a _"*"_ and let me know, if it works.
 
 # Prequisites for the examples