initial add S20 keyboard into polycarbdiet folder (#11865)

keyboards/polycarbdiet/s20/readme.md

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+# S20
+
+![S20](https://i.imgur.com/RvOACqR.jpg?1)
+
+The S20 keyboard is a FR4 sandwich keyboard which was orginally designed to add extra functionality to a user's current keyboard setup.
+
+* Keyboard Maintainer: [polycarbdiet](https://github.com/polycarbdiet)
+* Hardware Supported: S20 - FR4 sandwich MacroPad/NumberPad
+* Hardware Availability: Custom, limitied quantaties. 
+
+Make example for this keyboard (after setting up your build environment):
+
+    make polycarbdiet/s20:default
+
+Flashing example for this keyboard:
+
+    make polycarbdiet/s20:default:flash
+
+**Reset Method:** 
+- Press the `RESET` button on the under side of the PCB  
+
+**Bootloader Method:** 
+- Hold down the key located at `K00`, commonly programmed as `ESC`, while plugging in the keyboard.
+- Hold down the key located at `K00`, commonly programmed as `ESC`, and press the RESET button on the PCB.
+- Hold down the key located at `K42` and then press the key located at `K40`.
+
+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).