[Keyboard] Add Galatea TKL PCB. (#15290)

keyboards/studiokestra/galatea/readme.md

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+# Galatea
+
+TKL H87/88c compatible PCB with support for the most common layouts.
+
+* Keyboard Maintainer: [Studio Kestra](https://github.com/studiokestra/)
+* Hardware Supported: [studiokestra.ca/galatea](https://studiokestra.ca/galatea/)
+* Hardware Availability: In-Stock Sale (Dec 2021)
+* Rev1 firmware is used for Galatea PCBs with no RGB underglow.
+* Rev2 firmware is used for Galatea PCBs with RGB underglow. 
+
+## Bootload Sequence
+
+There are 3 ways to put the board in bootloader mode:
+
+- Hold the top-left key (typically `Esc`) while plugging in the USB cable, OR
+- While the PCB is plugged into the PC, press the physical `RESET` button on the back of the board, OR
+- With the default layout, toggle Layer 1 and press the `R` key. 
+
+## Compiling Firmware
+
+Make example for this keyboard (after setting up your build environment):
+
+    make studiokestra/galatea/rev1:default
+
+If no revision is specified, it will build rev1 firmware by 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).