doc: Update the ErgoDox & Atreus docs to mention teensy_loader_cli

The ErgoDox typically uses a Teensy, and the Atreus has the option to as well.
In both cases, the flashing procedure (whether through the IDE or CLI) will
require the `teensy_loader_cli` tool installed.

Fixes #479.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

doc/plugin/Hardware-EZ-ErgoDox.md

@@ -1,6 +1,8 @@
 # Kaleidoscope-Hardware-ErgoDox
 
-This is a plugin for [Kaleidoscope][fw], that adds hardware support for
+This is a plugin for [Kaleidoscope][fw], that adds hardware support for the
-the ErgoDox.
+ErgoDox. To be able to flash the firmware, one will need the [Teensy Loader
+CLI][teensy_cli] tool in addition to Arduino.
 
+ [teensy_cli]: https://www.pjrc.com/teensy/loader_cli.html
  [fw]: https://github.com/keyboardio/Kaleidoscope

doc/plugin/Hardware-Technomancy-Atreus.md

@@ -6,6 +6,11 @@ MCU, and the hand-wired variant from [FalbaTech][falba] with a Teensy2.
 
 PCBs prior to 2016, and the legacy teensy2 variants are not supported.
 
+To be able to flash the firmware, one will need the [Teensy Loader
+CLI][teensy_cli] tool in addition to Arduino, if using a Teensy. If using an A*
+MCU, the additional tool is not required.
+
+ [teensy_cli]: https://www.pjrc.com/teensy/loader_cli.html
  [fw]: https://github.com/keyboardio/Kaleidoscope
  [atreus]: https://atreus.technomancy.us/
  [falba]: https://falba.tech/