[Keyboard] Add scylla keyboard (#12720)

Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Ryan <fauxpark@gmail.com>
Co-authored-by: bstiq <qlebastard@outlook.com>

keyboards/bastardkb/scylla/readme.md

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+# Scylla
+
+A modern, low-profile split ergonomic keyboard
+
+* Keyboard Maintainer: [Bastard Keyboards](https://github.com/Bastardkb/)
+* Hardware Supported: elite-C V4
+* Hardware Availability: [Bastard Keyboards](https://bastardkb.com/)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make bastardkb/scylla: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).
+
+See the [keyboard build instructions](https://docs.bastardkb.com)
+
+
+## Important information regarding the reset
+
+If you modify this firmware, make sure to always have a RESET key that can be triggered using only the master side ! This way you ensure that you can always flash the keyboard, even if you mess up.
+
+Otherwise if you're stuck, open the case and reset manually by shorting Gnd and Rst, or pressing the RST button.