Add Sonoff DIY docs

Sonoff-DIY.md

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+Some new Sonoff devices support Sonoff DIY (https://github.com/itead/Sonoff_Devices_DIY_Tools). With Sonoff DIY a user has more control over the hardware features of the device and also allows for upgrading the firmware without additional hardware.
+
+## Compatible devices
+Currently the following devices officially support Sonoff DIY:
+- Sonoff Basic R3
+- Sonoff RF R3
+- Sonoff Mini
+
+As Sonoff DIY is enabled by connecting GPIO16 to Gnd it may well be possible that other Sonoff devices running eWelink 3.1 or higher will also support it.
+
+## Procedure
+The following procedure upgrades Sonoff eWelink software to Tasmota using a Windows laptop.
+- Open the device and remove the jumper labeled OTA if present
+- Power on device and connect to eWelink
+- Update firmware to at least 3.1 (mine came with 3.0)
+- Power off and install jumper
+- Download the Sonoff DIY package from Github at https://github.com/itead/Sonoff_Devices_DIY_Tools
+- Modify your laptop (windows 10) configuration to provide a Mobile hotspot with SSID sonoffDiy (case sensitive!) and password 20170618sn
+
+<img src="https://github.com/arendst/arendst.github.io/blob/master/media/w10mobile_hotspot.png" />
+
+- Power on and verify it pops up on the Mobile hotspot Settings page
+- Start ``\tool\tool_01DIY85(v1.0.0).exe``
+- It should find the device
+- Select it and play with ON and OFF to verify correct connection to Basic R3
+- Select Firmware flash
+- Select pre-compiled or own compiled binary smaller than 500k. Do NOT SELECT sonoff-minimal as it does not allow to change any setting.
+- Select the device in the flash pop-up and select OK
+- Tasmota will be uploaded an start running
+
+Now continue as if you just installed Tasmota on the device.
+
+### Clean up:
+- Quit DIY mode tool
+- Stop mobile hotspot
+- Power off device and remove OTA jumper as it is no longer needed and might interfere with future Tasmota features using GPIO16