Update Phantom readme

Added basic description of the keyboard and some build and configuration
instructions.

Also moved the RGB underlight modification instructions to the readme.

keyboards/phantom/pinout.txt

@@ -1 +0,0 @@
-For WS2812B LED strip support, connect DIN from strip to PE2 on Teensy (see reference image https://i.imgur.com/aDfNoHT.jpg)

keyboards/phantom/readme.md

@@ -1,4 +1,48 @@
 Phantom keyboard firmware
-======================
+=========================
 
-TODO: to be updated.
+A community-developed keyboard PCB designed to fit inside the case of a Filco Majestouch.
+
+See the [Deskthority wiki](https://deskthority.net/wiki/Phantom) for more information.
+
+## Bootloader
+
+The Phantom uses a [Teensy 2.0](https://www.pjrc.com/store/teensy.html) as a controller.
+
+The Teensy has a special bootloader that can be accessed by pressing the button on the Teensy PCB. It is also possible to use Boot Magic and Command to access the bootloader.
+
+To write the firmware to the Teensy use [Teensy loader](https://www.pjrc.com/teensy/loader.html).
+
+## Quantum MK Firmware
+
+For the full Quantum feature list, see the [documentation](https://docs.qmk.fm).
+
+## RGB underlight
+
+It is possible to connect a WS2812B LED strip to the Teensy for RGB underlight support.
+
+For this to work the DIN connection on the WS2812B strip should be soldered to PE2 on the Teensy (see reference image https://i.imgur.com/aDfNoHT.jpg).
+
+See [rgbmod](keymaps/rgbmod) for a keymap that utilizes the RGB underlight feature.
+
+## Building
+
+The Phantom allows for a huge amount of different layouts.
+
+Depending on which layout and keymap you would like to use, you will have to compile the firmware slightly differently. All of the commands should be run in the [keyboards/phantom](/keyboards/phantom) folder.
+
+### Custom keymaps
+
+To define your own keymap, copy one of the [existing keymap](keymaps) folders and give it the name of your keymap. Then check the [keymap documentation](https://docs.qmk.fm/Keymap.html) for details on how to modify the keymap.
+
+To make it easy to define keymaps for the most common layouts a few macros are provided.
+
+| Layout          | Macro                     |
+| --------------- | ------------------------- |
+| Winkey ANSI     | `KEYMAP()`                |
+| Winkeyless ANSI | `KEYMAP_WINKEYLESS()`     |
+| Winkey ISO      | `KEYMAP_ISO()`            |
+| Winkeyless ISO  | `KEYMAP_ISO_WINKEYLESS()` |
+| 7BIT            | `KEYMAP_7BIT()`           |
+
+To build the firmware with a custom keymap, run `make <keymap name>`