Added README for EV3D4 (#254)

* Added README for EV3D4

* Added README for EV3D4 (address comments from code review)

* Added README for EV3D4 (fix backticks)

* Added README for EV3D4 (keep lines under 80 characters)

Author: dwalton76

demo/EV3D4/README.md

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+# EV3D4
+EV3D4 is designed to look like R2-D2 from Star Wars. There are two options for
+controlling EV3D4. The first is to use the IR remote to send commands to the IR
+sensor, run EV3D4RemoteControl to use this method. The second means of
+controlling EV3D4 is via a web browser, run EV3D4WebControl to use this method.
+
+**Building instructions**: https://www.lego.com/en-us/mindstorms/build-a-robot/ev3d4
+
+### EV3D4RemoteControl
+If you are using EV3D4RemoteControl the only other file you need from
+`demo/EV3D4/` is EV3D4.py. The other files are only needed by EV3D4WebControl.
+
+EV3D4RemoteControl creates a child class of ev3dev/helper.py's
+RemoteControlledTank.
+
+
+### EV3D4WebControl
+EV3D4WebControl creates a child class of ev3dev/helper.py's WebControlledTank.
+The WebControlledTank class runs a web server that serves the web pages,
+images, etc but it also services the AJAX calls made via the client. The user
+loads the initial web page at which point they choose the "Desktop interface"
+or the "Mobile Interface".
+
+Desktop Interface - The user is presented with four arrows for moving forwards,
+backwards, spinning clockwise or spinning counter-clockwise. Two additional
+buttons are provided for controlling the medium motor. There are two sliders,
+one to control the speed of the tank and the other to control the speed of the
+medium motor.
+
+Mobile Interface - The user is presented with a virtual joystick that is used
+to control the movements of the robot. Slide your thumb forward and the robot
+moves forward, slide it to the right and the robot spins clockwise, etc. The
+further you move the joystick towards the edge of the circle the faster the
+robot moves. Buttons and a speed slider for the medium motor are also provided.
+
+Both interfaces have touch support so you can use either Desktop or Mobile from
+your smartphone. When the user clicks/touches a button some jQuery code will
+fire off an AJAX call to let the EV3D4 web server know what the user clicked or
+where the joystick is if using the Mobile Interface. The web server in
+WebControlledTank services this AJAX call and adjust motor speed/power
+accordingly.
+
+You can see a demo of the web interface below. Note that the demo is on a
+simple Tank robot, not EV3D4, but that doesn't really matter as EV3D4 is also
+just a Tank robot.
+
+**Demo Video**: https://www.youtube.com/watch?v=x5VauXr7W4A