feature: add basic doc about how a keyboard works on USB

This comes from the discussion on #520

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+# How keys are registered, and interpreted by computers
+
+In this file, you can will learn the concepts of how keyboards work over USB,
+and you'll be able to better understand what you can expect from changing your
+firmware directly.
+
+## Schematic view
+
+Whenever you type on 1 particular key, here is the chain of actions taking
+place:
+
+``` text
++------+         +-----+       +----------+      +----------+     +----+
+| User |-------->| Key |------>| Firmware |----->| USB wire |---->| OS |
++------+         +-----+       +----------+      +----------+     |----+
+```
+
+This scheme is a very simple view of what's going on, and more details follow
+in the next sections.
+
+## 1. You Press a Key
+
+Whenever you press a key, the firmware of your keyboard can register this event.
+It can register when the key is pressed, held and released.
+
+This usually happens with a [periodic scan of key presses with a frequency around 100 hz](https://github.com/benblazak/ergodox-firmware/blob/master/references.md#typical-keyboard-information).
+This speed often is limited by the mechanical key response time, the protocol
+to transfer those key presses (here USB HID), and by the software it is used in.
+
+## 2. What the Firmware Sends
+
+The [HID specification](http://www.usb.org/developers/hidpage/Hut1_12v2.pdf)
+tells what a keyboard can actually send through USB to have a chance to be
+properly recognised. This includes a pre-defined list of keycodes which are
+simple numbers from `0x00` to `0xE7`. The firmware assigns a keycode to each
+key of the keyboard.
+
+The firmware does not send actually letters or characters, but only keycodes.
+Thus, by modifying the firmware, you only can modify what keycode is sent over
+USB for a given key.
+
+## 3. What the Operating System Does
+
+Once the keycode reaches the operating system, a piece of software has to have
+it match an actual character thanks to a keyboard layout. For example, if your
+layout is set to QWERTY, a sample of the matching table is as follow:
+
+``` text
+| keycode | character |
+|---------+-----------|
+|    0x04 | a/A       |
+|    0x05 | b/B       |
+|    0x06 | c/C       |
+|     ... | ...       |
+|    0x1C | y/Y       |
+|    0x1D | z/Z       |
+|     ... | ...       |
+|---------+-----------|
+```
+
+## Back to the firmware
+
+As the layout is generally fixed (unless you create your own), the firmware can
+actually call a keycode by its layout name directly to ease things for you.
+
+This is exactly what is done here with `KC_A` actually representing `0x04` in
+QWERTY. The full list can be found in `keycode.txt`.
+
+## List of Characters You Can Send
+
+Putting aside shortcuts, having a limited set of keycodes mapped to a limited
+layout means that **the list of characters you can assign to a given key only
+is the ones present in the layout**.
+
+For example, this means that if you have a QWERTY US layout, and you want to
+assign 1 key to produce `€` (euro currency symbol), you are unable to do so,
+because the QWERTY US layout does not have such mapping. You could fix that by
+using a QWERTY UK layout, or a QWERTY US International.
+
+You may wonder why a keyboard layout containing all of Unicode is not devised
+then? The limited number of keycode available through USB simply disallow such
+a thing.
+
+## How to (Maybe) Enter Unicode Characters
+
+You can have the firmware send *sequences of keys* to use the [software Unicode
+Input
+Method](https://en.wikipedia.org/wiki/Unicode_input#Hexadecimal_code_input) of
+the target operating system, thus effectively entering characters independently
+of the layout defined in the OS.
+
+Yet, it does come with multiple disadvantages:
+
+ - Tied to a specific OS a a time (need recompilation when changing OS);
+ - Within a given OS, does not work in all software;
+ - Limited to a subset of Unicode on some systems.