Added some information to README

Author: jakobend

README.md

@@ -1,2 +1,55 @@
-# gcspark
-Low-cost USB adapter for Gamecube controllers
+# GCSpark
+
+The goal of this project is to create a low-cost, easily buildable USB adapter
+for Gamecube input peripherals.
+
+## Why?
+
+I own a Wii dance mat and want to play [StepMania](https://www.stepmania.com)
+on it.
+
+Existing commercial solutions tend to be expensive, and cheaper clones are
+known to have issues. Previously published DIY projects use overpowered
+(and thus expensive) microcontrollers, are hard to build for inexperienced
+people and/or do not implement rumble.
+
+## Overview
+
+I decided to choose the [Digispark](http://digistump.com/products/1) Attiny85
+development board as the base of this project. Clones of it are affordable and
+very easy to source. The integrated USB plug/port simplifies wiring, and using
+it as an USB device via [V-USB](https://www.obdev.at/products/vusb/index.html)
+is well-documented. Since it is a development board, no ISP is required to
+program it.
+
+Gamecube controllers expect both a 5V and a 3.3V supply and operate on 3.3V
+logic level. To cut down on the amount of external components, a breakout board
+for the AMS1117 LDO is used to supply 3.3V. The bidirectional level shifting
+will be done using a traditional circuit based around the 2N7000 MOSFET, which
+is hopefully able to switch at the required 1 MHz.
+
+Instead of emulating a USB PID (physical interface device) for rumble support,
+which other developers seem to have no luck with, the Microsoft XInput protocol
+will be used. Plain USB HID support may be added in the future, either
+switchable (if memory limitations allow it) or as alternative firmware.
+
+## Cost Estimation
+
+The following estimation is based on the lowest AliExpress prices, including
+shipping to Europe, at time of writing. Some components are primarily available
+in bulk, and the PCS column shows the bulk size.
+
+| Count | Component               | Est. Cost | PCS |
+|-------|-------------------------|-----------|-----|
+| 1     | Digispark (clone)       | $1.30     | 1   |
+| 1     | AMS1117 breakout board  | $1.20     | 5   |
+| 1     | GC extension cable      | $2.00     | 1   |
+| 1     | Perfboard               | $1.00     | 5   |
+| 1     | 2N7000 MOSFET (generic) | $0.80     | 20  |
+| 2     | 10K resistors           | $0.70     | 100 |
+|       | Sum                     | $7.00     |     |
+
+The extension cable is only required for the female connector. If one is willing
+to cut off the cable of their controller and hardwire it into the adapter, it
+can be omitted. Shoving solid wire (e.g. Dupont connectors) in the male GC
+connector may also work as a short-term solution.