Make a note on the classes that they aren't built in and provide a link to installation instructions.

Author: tannewt

README.rst

@@ -1,5 +1,5 @@
 Adafruit CircuitPython BusDevice
-==============================
+================================
 
 .. image:: https://readthedocs.org/projects/adafruit-circuitpython-busdevice/badge/?version=latest
     :target: https://circuitpython.readthedocs.io/projects/bus_device/en/latest/
@@ -10,6 +10,20 @@ on a bus easy. For example, they manage locking the bus to prevent other
 concurrent access. For SPI devices, it manages the chip select and protocol
 changes such as mode. For I2C, it manages the device address.
 
+.. _bus_device_installation:
+
+Installation
+-------------
+
+This library is **NOT** built into CircuitPython to make it easy to update. To
+install it either follow the directions below or :ref:`install the library
+bundle <bundle_installation>`.
+
+To install:
+
+#. Download and unzip the `latest release zip <https://github.com/adafruit/Adafruit_CircuitPython_BusDevice/releases>`_.
+#. Copy the unzipped `adafruit_bus_device` to the ``lib`` directory on the ``CIRCUITPY`` or ``MICROPYTHON`` drive.
+
 API
 ---
 .. toctree::

adafruit_bus_device/i2c_device.py

@@ -28,6 +28,9 @@ class I2CDevice:
     :param ~nativeio.I2C i2c: The I2C bus the device is on
     :param int device_address: The 7 bit device address
 
+    .. note:: This class is **NOT** built into CircuitPython. See
+      :ref:`here for install instructions <bus_device_installation>`.
+
     Example:
 
     .. code-block:: python

adafruit_bus_device/spi_device.py

@@ -28,6 +28,9 @@ class SPIDevice:
     :param ~nativeio.SPI spi: The SPI bus the device is on
     :param ~microcontroller.Pin chip_select: The chip select pin
 
+    .. note:: This class is **NOT** built into CircuitPython. See
+      :ref:`here for install instructions <bus_device_installation>`.
+
     Example:
 
     .. code-block:: python