add project README (hardware files are upcoming)

README.md

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+purpose
+=======
+
+the name of the device says it all: "USB cable tester".
+it will test what kind of USB cable it is by checking which signals on the connectors are inter-connected by the cable.
+this helps identifying if a cable can transmit data or just deliver power.
+Apple's lightning connector is also supported although it is not strictly a USB connector.
+
+background
+----------
+
+micro-USB (USB micro-B) cables come with each new gadget to power it, or recharge its battery.
+these cable are not actual USB cables (which can transfer USB data), but only deliver power.
+the USB connectors are just used because it is convenient, and an ubiquitous power source.
+but you don't want to confuse them with actual USB cables supporting data transfer.
+sadly there are often no visible differences between a USB data cable and one just to deliver power.
+the USB cable tester is a tool to identify the USB cables.
+afterwards don't forget to label them afterwards to not get tricked in the future and stop wasting debugging time.
+
+since I would built a device to test USB cables, why stop at just testing if it has data lines.
+it can also test if the cable is shielded, supports USB 3.x, and even other USB connectors.
+this becomes even more interesting for USB-C cables since they have plenty of signals, some optional, and this is not identifiable just by looking at the cable.
+
+connectors
+----------
+
+the USB cable tester has the following connectors:
+- 2x USB type-A, supporting USB 3.x
+- 2x USB type-C, with all 24 pins
+- 1x USB type-B, supporting USB 3.x
+- 1x USB mini-B, USB 2.0 only
+- 1x USB micro-B, supporting USB 3.x
+- 1x Apple lightning
+
+these are the most commonly used USB connectors.
+the Apple lightning socket has been added because it is also a common charging connector, and there were enough free pins to also test this.
+there are two USB type-A connectors because some rare devices also use it (for example PCIe extension boards), although this should be a host-side connector.
+some Y-cables also use a second type-A host connector for devices requiring additional power.
+any combination of the connectors and signals can be tested (a list of know cables is built in the firmware).
+
+usage
+-----
+
+- plug all ends of the USB cable to test in the corresponding sockets
+- choose if the USB cable tester should be powered through its USB port or the battery using the switch
+- press on the RESET/START button
+- the result of the cable test will be displayed on the LCD and/or OLED screen
+
+if you connect the USB cable tester over USB (using the dedicated micro-B connector) to a computer, it will appear as USB device with a serial port.
+you can connect to it using a terminal (the baud rate does not matter) and have a console for more detailed tests and results.
+this is useful for cables with could not be identified, often because they are faulty.
+more tests are available on the serial prompt to identify the issues with the cable.
+just type "help" to get the available commands.
+
+limitations
+-----------
+
+the USB cable tester only tests if the signals are connected.
+it does not test:
+
+- the resistance of the cable (useful for cables used to delivery high currents)
+- if the shield is actually shielding the data cables or is just a wire (the impedance would need to be measured, which is complicated)
+- if the data line pairs are twisted (the impedance would need to be measured, which is complicated)
+
+I did not include the mini-A, micro-A, mini-AB, and micro-AB connectors since they are quite uncommon.
+
+firmware
+========
+
+binary
+------
+
+ready to flash firmware binaries are in the `release` folder
+
+source code
+-----------
+
+the firmware source code for this device is in the `firmware` folder.
+if you got this project using git, you need also check out this submodule using `git submodule init`.
+the source code in under GPL version 3 license.
+for more details check the `README` file on the `firmware` folder.
+this will also provide informations for firmware development.
+
+hardware
+========
+
+production
+----------
+
+the pdf schematic, board layer gerbers, and various other output files are in the `release` folder.
+
+design files
+------------
+
+the source design files are in the `hardware` folder.
+they are under the CERN Open Hardware Licence version 1.2.
+for more information check the `README` file on the `hardware` folder.
+