Update README.adoc

Author: karlsoderby

README.adoc

@@ -1,5 +1,13 @@
+
 = Reference-en
 
+[NOTE]
+====
+This repository is no longer actively maintained. The Arduino Language Reference has moved to the https://github.com/arduino/docs-content-reference[`docs-content-reference`] repository.
+
+The content is now rendered at https://docs.arduino.cc/language-reference/
+====
+
 Reference-en is the repo for the Language Reference documentation of the Arduino syntax in **English**.  
 All the Reference terms files are in AsciiDoc format.