add instructions for perms

README.md

@@ -1,8 +1,26 @@
-# Onyx Keyboard Firmware #
+# Onyx Keyboard Firmware
 
 This section intentionally left blank
 
-## Setup ##
+## Setup
+
+If you're running Linux, make sure you are part of the group for the character
+device that the board is connected to.
+
+For example, the device in bootloader mode appears as `/dev/ttyACM0`:
+
+```bash
+crw-rw---- 1 root uucp 166, 0 Jan 26 00:40 /dev/ttyACM0
+```
+
+You will need to add yourself to the group if you aren't part of it already, and
+log back in:
+
+```bash
+sudo usermod -a -G uucp $(whoami)
+```
+
+Then, follow the rest below:
 
 ```bash
 # Install toolchains