[Keyboard] Add Slime88 (#19054)

Co-authored-by: Joel Challis <git@zvecr.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/zykrah/slime88/readme.md

@@ -0,0 +1,27 @@
+# Slime88
+
+![Slime88](https://i.imgur.com/DYNJZFHh.png)
+
+A Hotswap Multi-layout F13 TKL PCB (h88c compatible) powered by RP2040.
+
+* Keyboard Maintainer: Zykrah
+* Hardware Supported: Slime88 rev1 PCB
+* Hardware Availability: [SLIME88](https://github.com/zykrah/slime88)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make zykrah/slime88:default
+
+Flashing example for this keyboard:
+
+    make zykrah/slime88:default:flash
+
+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).
+
+## Bootloader
+
+Enter the bootloader in 3 ways:
+
+* **Bootmagic reset**: Hold down the key at (0,0) in the matrix (usually the top left key or Escape) and plug in the keyboard
+* **Physical reset button**: Briefly short the `RST` and `GND` pads on the SWD header twice, or short the `BOOT` header and plug in keyboard
+* **Keycode in layout**: Press the key mapped to `QK_BOOT` if it is available