Added information about SPI on the RPi4

The RPi4 has a higher core frequency and can not be underclocked to 250MHz like the RPi3. Instead its default frequency is 500MHz, which is compatible with SPI. However, this value is changed by the dynamic frequency clocking. The minimum frequency has to be raised to prevent SPI desyncs.

Author: jasminhacker

library/README.rst

@@ -151,6 +151,15 @@ following line to /boot/config.txt and reboot.
 
         core_freq=250
 
+On a RPi 4 its dynamic frequency clocking has to be disabled, since it will
+desync the SPI clock. Do this by adding this line to
+``/boot/config.txt``. (``core_freq`` does not have to be changed, since
+the default value of 500MHz is SPI compatible)
+
+::
+
+        core_freq_min=500
+
 SPI requires you to be in the ``gpio`` group if you wish to control your
 LEDs without root.