[Keymap] added hhkb layout for tada68 (#6027)

keyboards/tada68/keymaps/hhkb68/README.md

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+# HHKB68
+Layout to closely match the primary and function layers of the HHKB.

keyboards/tada68/keymaps/hhkb68/readme.md

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+# Generated Keymap Layout
+
+This layout was generated by the QMK API. You can find the JSON data used to
+generate this keymap in the file layers.json.
+
+To make use of this file you will need follow the following steps:
+
+* Download or Clone QMK Firmware: <https://github.com/qmk/qmk_firmware/>
+* Extract QMK Firmware to a location on your hard drive
+* Copy this folder into %s
+* You are now ready to compile or use your keymap with the source
+
+More information can be found in the QMK docs: <https://docs.qmk.fm>