Update mitch layout for new keymaps

keyboards/planck/keymaps/mitch/readme.md

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+## Flashing Keyboard
+
+1. Install `dfu` tools:
+
+    brew tap osx-cross/avr
+    brew install avr-libc
+    brew install dfu-programmer
+
+2. Move to this directory.
+3. Hit the reset button on the keyboard.
+4. run `make dfu`.
+
+## The Keymap
+
+This keymap is designed for a rev3 Planck Keyboard.
+
+The default layer is QWERTY. The raise layer has a ten key on the right
+and common programming punctuation on the left. The lower layer provides the
+rest of the symbols, mostly mapped with the ten key numbers.
+
+The normal right shift key uses the `MT` macro to trigger Enter on tap and right
+shift when held.
+
+This keymap sets the `PREVENT_STUCK_MODIFIERS` flag to avoid the occasional WTF
+moments when using a modifier keys and accidentally releasing them after moving
+to a new layer.