add hint about the device tree overlay to README.md

README.md

@@ -10,6 +10,8 @@ Execute `make` to compile the kernel module and then load the compiled module vi
 
 To use this module, you will need either a DS1307 oder DS1337 rtc module that is attached to your computer via i2c. To compile it, you will need gcc, make and the kernel-headers for your currently running kernel. Usually you will be able to install the kernel headers through a package called `kernel-headers`, `linux-headers` or `raspberrypi-kernel-headers` in the case of Raspbian. The name of the package largely depends on your distribution.
 
+For the Raspberry PI you will also need to enable the i2c-rtc and ds1307 device tree overlays. To do that, put `dtoverlay=i2c-rtc,ds1307` into your `/boot/config.txt`.
+
 ### Installing
 
 TBD