doc: kconfig: add exemption for busses in select recommended practices

Add the exemption for busses to the recommended practices
for select.

Signed-off-by: Benedikt Schmidt <benedikt.schmidt@embedded-solutions.at>

doc/build/kconfig/tips.rst

@@ -315,6 +315,16 @@ In summary, here are some recommended practices for ``select``:
 - Select simple helper symbols without prompts and dependencies however much
   you like. They're a great tool for simplifying Kconfig files.
 
+- An exemption are busses like I2C and SPI, and following the same thought
+  process things like MFD as well. Drivers on these busses should use
+  ``select`` to allow the automatic activation of the necessary bus drivers
+  when devices on the bus are enabled in the devicetree.
+
+.. code-block:: kconfig
+
+   config ADC_FOO
+      bool "external SPI ADC foo driver"
+      select SPI
 
 (Lack of) conditional includes
 ******************************