[Keyboard] Adds Elongate Revision Delta PCB support (#15498)

Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: James Young <18669334+noroadsleft@users.noreply.github.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

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+# Acheron Elongate rev. Beta QMK firmware repository
+
+![](https://i.imgur.com/G9LESkUh.jpg)
+
+* Keyboard Maintainer: [Gondolindrim](https://github.com/Gondolindrim)
+* Hardware Supported: [open-source PCB](https://github.com/AcheronProject/Elongate) powered by STM32F072 microcontroller
+* Hardware Availability: the revision beta of Elongate was never publicly sold.
+
+## Introduction
+
+This is the QMK firmware repository for the Elongate PCB revision Beta and pre-revision Beta versions, which was the initial revision PCB for this keyboard; hence it was never sold publicly.
+
+The Elongate PCB is an Open-Hardware-guideline-compliant PCB which files can be found at [this repository](https://github.com/AcheronProject/Elongate). Its designer and maintainer is [Gondolindrim](https://github.com/Gondolindrim).
+
+## How to flash
+
+### Enter bootloader
+
+The DFU state in the bootloader can be accessed 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**: press the button on the front of the PCB.
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available.
+
+## Compile firmware
+
+You can build the default layout firwmare for Elongate after setting up your build environment by using:
+
+    make acheron/elongate/beta:default
+
+After entering DFU state in your PCB, you can download the binary file generated through `dfu-util` or a GUI like QMK Toolbox.
+
+To directly flash the PCB after it is put into a DFU state, use
+
+    make acheron/elongate/beta: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).
+
+## PCB Documentation
+
+See the [AcheronDocs](https://acheronproject.com/pcbs/elongate/) page for the ElongatePCB full documentation. You can also check the KiCad PCB files at the [Elongate GitHub repository](https://github.com/AcheronProject/Elongate).
+
+Before using the files for personal or commercial use, please read the [Acheron Open-Hardware License V1.4](https://acheronproject.com/AOHL14/) under which the Elongate PCB is published.