Add Zero4U board information

src/en/overlay/zero4u.md

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+<!--
+---
+name: Zero4U
+class: board
+type: usb hub
+formfactor: pHAT
+image: 'zero4u.png'
+manufacturer: UUGear s.r.o.
+description: 4-port USB hub for Raspberry Pi Zero
+url: http://www.uugear.com/product/zero4u/
+buy: http://www.uugear.com/product/zero4u/
+pincount: 40
+eeprom: no
+power: 5v
+-->
+# Zero4U
+
+Zero4U is a 4-port USB hub for Raspberry Pi Zero, and it can be mounted to Raspberry Pi Zero back-to-back. The 4 pogo pins on the back will connect the PP1, PP6, PP22 and PP23 testing pads on Raspberry Pi Zero, hence no soldering will be needed to make it work.
+
+The USB hub will take power directly from your Raspberry Pi Zero, so you don’t need to power the USB hub separately. However you can use the JST XH2.54 connector on board as the alternative power input.
+
+There are one blue LED on board as the power indicator, and four white LED aside the USB ports as transaction indicators.
+
+This USB hub can also work with other types of computer. You can use a USB cable to connect the mini-USB port on board to a USB port on the computer.