[Keymap] Add "shwilliam" Preonic keymap (#9409)

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

keyboards/preonic/keymaps/shwilliam/readme.md

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+# [@shwilliam](https://github.com/shwilliam)'s Preonic Layout
+
+![shwilliam's preonic layout image](https://user-images.githubusercontent.com/38357771/84585088-dc536380-adc0-11ea-8378-6fb8ffbc6a8d.png)
+
+## Overview
+
+- Shift, backspace, raise and lower on thumbs
+- Arrows on Vim keys
+- Function arrow key (`=>`)
+- Common parens one key from home-row
+- Brightness/volume control
+- Tap caps-lock for escape, hold for left control and left shift
+- Alternative punctuation
+  - Corner brackets
+  - Double-angle brackets
+  - Lenticular brackets
+- Select to left and right keys
+- Emoji layer
+
+## Layers
+
+### QWERTY
+
+- Base layer; similar to default Preonic keymap
+
+### Raise
+
+- Mostly shifted QWERTY layer with alternative actions
+- Brightness control
+
+### Lower
+
+- Common parentheses
+- Arrows on Vim keys
+- Mouse controls on WASD
+- Volume control
+
+### Emoji
+
+- Emoji layout based on mnemonics