Add documentation of udev rules for hwp-pcu (#622)

* add documentation of udev rules for hwp-pcu

* Wrap text to 80 characters

* Edit udev rule description

---------

Co-authored-by: Brian Koopman <brian.koopman@yale.edu>

docs/agents/hwp_pcu.rst

@@ -18,6 +18,28 @@ motors the same.
     :func: make_parser
     :prog: python3 agent.py
 
+Dependencies
+------------
+
+The PCU device shows up on the system as ``/dev/ttyACMx`` where ``x`` is not
+guaranteed to be consistent. To solve this issue we should create a udev rule
+for the PCU device. Establish the udev rule by creating a file in
+``/etc/udev/rules.d/`` with the following contents::
+
+  SUBSYSTEM=="tty", ATTRS{idVendor}=="2a19", ATTRS{idProduct}=="0c02", MODE="0666", SYMLINK="PCU"
+
+.. note::
+    If multiple PCU devices are connected to the same computer you will want to
+    include the serial number in the rules, which you can add with
+    ``ATTRS{serial}=="<serial number of PCU>"``.
+
+If the PCU was connected to the computer already you will need to unplug and
+replug it for the udev rule to properly recognize the device, create the
+symlink, and set the permissions appropriately. Once you complete this step the
+device will be recongized on reboot, and the udev rules will not need to be
+reinstalled unless you add a new device. The PCU should now be available at
+``/dev/PCU``.
+
 Configuration File Examples
 ---------------------------