diff --git a/hardware/README.md b/hardware/README.md
new file mode 100644
index 000000000..3043da2f7
--- /dev/null
+++ b/hardware/README.md
@@ -0,0 +1,9 @@
+### Cores have been moved
+Originally, the main Arduino repository contained Arduino
+platforms/cores in this directory. Since then, these have been moved to
+their own repositories instead (see commit 2e98854 and d6b40d0).
+
+The official Arduino cores can now be found here:
+ - https://github.com/arduino/ArduinoCore-avr
+ - https://github.com/arduino/ArduinoCore-sam
+ - https://github.com/arduino/ArduinoCore-samd