add README.md into keyboards/helix/rev2/keymaps/led_test/ (#2582)

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+# LED test Keymap
+
+## Layout
+
+The layout is the same as default.
+
+## Feature
+
+* OLED is enabled.
+* LED backlight is enabled and always lit.
+* The lighting color of LED changes periodically. Red, Green and Blue.
+
+The user can check whether the LED is lit or not.
+
+## Compile
+
+```
+$ cd qmk_firmware
+$ make helix:led_test
+```
+
+## Flash QMK Firmware
+
+Execute the 'make' command and press the reset switch on the keyboard.
+```
+$ make helix:led_test:avrdude
+```