Create README in hardware folder

The contents of the `hardware/` folder were moved, but it wasn't obvious
straight away from looking at the repository. This makes it immediately
obvious for anyone looking for something in this folder.

Author: polymetric

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