Add MacOS Picotool upload help (earlephilhower#1355)

Fixes earlephilhower#1344

Author: earlephilhower

docs/install.rst

@@ -143,7 +143,13 @@ So for the first sketch you will need to rebuild (with the ``Upload Method->Pico
 
 After the initial upload, as long as the running binary was built using the ``Picotool`` upload method, then the normal upload process should work.
 
-Note that for Ubuntu and other Linux operating systems you may need to add the following lines to a new `udev` config file(``99-picotool.rules``) to allow normal users to access the special USB device the Pico exports:
+Under MacOS, it may be necessary to install the USB support libraries from a command terminal before the ``Picotool`` upload method can be used:
+
+.. code::
+
+        brew install libusb
+
+For Ubuntu and other Linux operating systems you may need to add the following lines to a new `udev` config file(``99-picotool.rules``) to allow normal users to access the special USB device the Pico exports:
 
 .. code::