Added Button Page

docs/helios64/button.md

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+## Onboard Button
+
+![!Button location](img/button/button_location.jpg)
+
+### Power Button
+
+Helios64 board provides a POWER push button (PWR BTN) to power on/off the system.
+This button is connected to PMIC RK808-D.
+
+Short press (~1 second) will turn on the system when the current state is power off. On Linux, this will inform kernel to perform gracefull shutdown.
+
+Long press (~4 seconds **TBC**) will signals PMIC to cut off the power in case of system not responding (Force power off).
+
+This button also routed to Front Panel, so user don't need to open the case to power on/off the system.
+
+### Reset Button
+
+Helios64 board provides a RESET push button (RST BTN) to hard reset the board.
+
+This button also routed to Front Panel, so user don't need to open the case to hard reset the system.
+
+### Recovery Button
+
+User can enter recovery mode by pressing this button during boot up (bootloader stage). U-Boot will read this button and switch the USB type C into USB Mass Storage device with eMMC as the storage.
+User then can use Balena Etcher or dd to write OS image into eMMC.
+
+On Linux, this button behave as user button and when pressed will emit **BTN_0** keycode.
+
+This button also routed to Front Panel, so user don't need to open the case to enter Recovery Mode.
+
+
+## Front Panel Button
+
+For button location on front panel please refer to [Front Panel](/helios64/front-panel/#the-front-panel-description).
+
+### USR2 Button
+
+There is additional user button on Front Panel board located on [P2](/helios64/front-panel/#wiring-diagram). On Linux, this button when pressed will emit **BTN_1** keycode.
+
+
+## References
+[input-event-codes.h](https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/plain/include/uapi/linux/input-event-codes.h)