Add Smith and Rune Magnus (#22153)

Co-authored-by: Less/Rikki <86894501+lesshonor@users.noreply.github.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Duncan Sutherland <dunk2k_2000@hotmail.com>

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+# Smith and Rune Magnus
+
+![magnus](https://imgur.com/bemM7ZZh.jpg)
+
+## Introduction
+
+This is the QMK Firmware repository for the Magnus, a keyboard designed by [Smith and Rune](https://smithrune.com/) and PCBs designed by [Gondolindrim](http://github.com/Gondolindrim), who is also the firwmare maintainer.
+
+The Magnus keyboard has two PCB versions: a solderable version (m75s) and a hotswap version (m75h). While M75S has support for a myriad of alternative layouts, including a 7U bottom row and ISO, M75H only supports full and split backspace alternatives. Also, while M75S supports in-switch single-color LEDs and RGB LEDs for underglow light, M75H only supports RGB underglow.
+
+As of July 2023, the Magnus groubuy is already finished.
+
+## 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, escape in the default keymap) and plug in the keyboard;
+* **Physical reset button**: press the button on the front of the PCB, next to caps lock, for at least five seconds;
+* **Keycode in layout**: Press the key mapped to `QK_BOOT`; in the default layout, that is top left key ('escape') in layer 1.
+
+### How to compile and flash
+
+After checking what PCB you own and setting up your build environment, you can compile the Magnus default keymap by using one of the options below.
+
+    make smithrune/magnus/m75s:default
+    make smithrune/magnus/m75h:default
+
+And use `dfu-util` in the command line or through a GUI like QMK toolbox to upload the firmware to the PCB. To directly flash the PCB after it is put into a DFU state, use:
+
+    make smithrune/magnus/m75s:default:flash
+    make smithrune/magnus/m75h: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).