Update README with usage information

Author: moonfloof

README.md

@@ -11,14 +11,54 @@ and different from my usual screen capture of FL Studio.
 
 ![](./screenshot.png)
 
-## Planned Features
-
-* [x] Load multiple audio tracks
-* [x] Render waveforms to screen
-* [x] Load dynamically from JSON
-* [x] Specify colours and titles for each track
-* [x] Change display size via config
-* [x] Render individual frames to PNG
-* [x] Handle errors properly!!!
-* [x] Change columns depending on aspect ratio (eg. for vertical video)
-* [ ] Render entire sequence to MP4
+## Usage
+
+```
+multitrack-visualizer.exe [OPTIONS]
+
+Options:
+  -s, --song /path/to/song.json       JSON config file for all tracks, colours, and audio files
+  -w, --window /path/to/window.json   JSON config file for size and scaling of the output video file
+  -h, --help                          Print help
+  -V, --version                       Print version
+```
+
+## JSON Config Format
+
+### Song.json
+
+* `channels`, an array of objects, where:
+  * `name` is the channel name, displayed on screen
+  * `file` is a path name to the audio file, and
+  * `colour` contains the Red, Green, and Blue colour values (0-255).
+* `video_file_out` is a path name to the video that will be output.
+
+```json
+{
+  "channels": [
+    {
+      "name": "Channel Name",
+      "file": "/path/to/audio-file.wav",
+      "colour": [0, 2, 255]
+    }
+  ],
+  "video_file_out": "/path/to/output.mp4"
+}
+```
+
+### Window.json
+
+* `width` and `height` are the base resolution for the video file.
+* `scale` is how many times the resolution should be scaled (integer).
+  * For example, the default is 480x270 at a scale of 4, which means the final
+    output resolution is 1920x1080.
+* `frame_rate` the frame rate of the output video
+
+```json
+{
+  "width": 480,
+  "height": 270,
+  "scale": 4,
+  "frame_rate": 30
+}
+```