[Keyboard] Waffling60 - readme update (#16707)

Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: 4pplet <4pplet@protonmail.com>
Co-authored-by: 4pplet <stefan.ess@gmail.com>

keyboards/4pplet/waffling60/readme.md

@@ -11,5 +11,12 @@ Make example for this keyboard (after setting up your build environment):
 
     make 4pplet/waffling60/rev_a:default
     make 4pplet/waffling60/rev_b:default
+    make 4pplet/waffling60/rev_c:default
 
 See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).
+
+How to enter bootloader (DFU):
+* Tap the physical reset button or short the reset-header (labled RESET) on tha back of the PCB for keyboard to enter DFU. When in DFU, it's ready to flash the firmware.
+
+Alternative option if the firmware is already pre-flashed:
+* Unplug your keyboard, hold down the Spacebar and B at the same time, plug in your keyboard and wait a second before releasing the keys. The keyboard will enter DFU and is ready to flash the firmware.