[Keyboard] Add BLASTER75 keyboard (#16887)

Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/blaster75/readme.md

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+# Blaster 75
+
+Blaster 75 is a 75% ANSI layout Custom Keyboard designed and manufactured by Altain. 
+
+* Keyboard Maintainer : ALTAiN
+* Hardware Supported : Blaster 75
+* Hardware Availability : Produced and Sold by ALTAiN
+* Mount : Top-Mount System
+* Typing Angle : 6 Degree
+* Front Height : 20.2mm
+
+## How to enter Bootloader Mode
+
+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.
+We strongly recommend this method. If the PCB is not soldered with switches at the moment, use anything that's metal (twizzers for example)
+to short the (0,0) position key ("ESC" in our PCB) while plugging the USB in. 
+
+* **Physical reset button**: Briefly press the button on the back of the PCB - some may have pads you must short instead
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available
+
+Make example for this keyboard (after setting up your build environment):
+
+    make blaster75:default
+
+Flashing example for this keyboard:
+
+    make blaster75: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).