document how to set udev rules manually.

doc/Quickstart.md

@@ -344,6 +344,25 @@ $ # for the NUCLE-F401RE
 $ openocd -f interface/stlink-v2-1.cfg -f target/stm32f4x.cfg
 ```
 
+> **NOTE(Linux)** If you get a permission error when running OpenOCD then you'll need to change the
+> udev rules for the SWD programmer you are using. To do that create the following file at
+> `/etc/udev/rules.d`.
+>
+> ``` console
+> $ cat /etc/udev/rules.d/99-st-link.rules
+> # ST-LINK v2
+> SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="3748", MODE:="0666"
+>
+> # ST-LINK v2-1
+> SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="374b", MODE:="0666"
+>
+> # NUCLEO-F401RE
+> SUBSYSTEMS=="usb", ATTRS{idVendor}=="0483", ATTRS{idProduct}=="374b", MODE:="0666"
+> ```
+>
+> With that file in place call the command `sudo udevadm control --reload-rules`. Then unplug and
+> re-plug your SWD programmer. That should fix the permission problem.
+
 You should see some output like this:
 
 ``` console