add more audio documentation (#505)

Co-authored-by: MegaMech <MegaMech@users.noreply.github.com>

docs/custom-audio.md

@@ -1,12 +1,35 @@
 # Custom Audio
-To replace a sequences you should only create a zip file with sequences to replace at the same path as mk64.o2r. Only wav, mp3, ogg and flac are supported.
+To replace a sequences you should only create a zip file with sequences to replace at the same path as mk64.o2r. Only wav, mp3, ogg and flac are supported. For define the loop you need to create a json with the same name but end with .json.
+the json look like this:
+```json
+{
+    "loop": {
+        "start": 0,
+        "end": 0
+    }
+}
+```
+The json file are optional. The unit of `start` and `end` is in pcm frames. You can use a tool like Audacity to find the correct values (it's call samples in Audacity). All properties avaible are:
+```json
+{
+    "name": "Name",
+    "author": "Author",
+    "date": "Date",
+    "loop": {
+        "start": 0,
+        "end": 0
+    }
+}
+```
+The `name`, `author` and `date` are optional. The `loop` is used to define the loop of the sequence. If you don't want to loop the sequence, you can just remove the `loop` property. `date` doesn't have any effect on the audio playback.
 ## Example:
 You want to change sound/sequences/main_menu then you just need to create a zip file with the following structure:
 ```
-textures_pack.zip
+audio_pack.zip
 └── sound
     └── sequences
         └── main_menu.mp3
+        └── main_menu.json
 ```
 
 ## Future plans