[Keyboard] Add Elrgo S to keyboards/handwired (#13893)

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

keyboards/handwired/elrgo_s/readme.md

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+# Elrgo S Split Keyboard
+
+This is a fork of the VOID Ergo S with QMK Configurator support.
+
+Elrgo S uses 2x Arduino Pro Micro (ATmega32U4) with 3D printed case.
+ 
+[Building guide, models for 3D printing](https://github.com/Eloren1/Elrgo_S)
+
+* Keyboard Maintainer: [Eloren](https://github.com/Eloren1)
+* Hardware Supported: Arduino Pro Micro
+* Hardware Availability: [Open Source](https://github.com/Eloren1/Elrgo_S)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make handwired/elrgo_s:default
+
+Flashing example for this keyboard:
+
+    make handwired/elrgo_s: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 2 ways:
+
+* **Physical reset button**: Briefly press the button through the hole on the left side of the case.
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available