Create song-pkg-format.md

Changes from Discord:

- remove meta.mer's #JACKET_FILE_PATH, since it's already stated that jacket.png/jpg is the file
- rename chart's #MOVIEOFFSET to #MOVIE_OFFSET

song-pkg-format.md

@@ -0,0 +1,51 @@
+Here is how a song in WacK is packaged.
+
+# Song folder Layout
+```
+<any name>/
+├─ 0.mer
+├─ 1.mer
+├─ 2.mer
+├─ 3.mer
+├─ meta.mer
+├─ jacket.png/.jpg
+├─ *.mp3/.ogg/.wav //SONG AUDIO
+```
+
+# meta.mer
+**This file must be present to define its folder as a song.** Everything here only dictates how the song is displayed in song select; none of these tags affect how the chart is played.
+
+```
+#TITLE <str>
+#RUBI <str>
+#ARTIST <str>
+#COPYRIGHT <str?>
+#GENRE <str?>
+#BPM <str>
+#PREVIEW_TIME <float>
+#PREVIEW_DURATION <float>
+```
+*`?` = optional
+
+# Chart .mer
+### Naming
+**Every charted .mer file must contain an integer that may be padded.** The first four, 0-3, correspond to in-game difficulty as follows:
+```
+0. Normal
+1. Hard
+2. Expert
+3. Inferno
+```
+
+### Tags
+`#LEVEL` (float): difficulty level  
+`#MUSIC_FILE_PATH` (str)  
+`#OFFSET` (float)  
+`#CLEAR_THRESHOLD` (float? 0-1)  
+`#AUTHOR` (str?)  
+`#PREVIEW_TIME` (float? seconds)  
+`#PREVIEW_DURATION` (float? seconds)  
+`#MOVIE_FILE_PATH` (str?)  
+`#MOVIE_OFFSET` (float? seconds)
+
+*`?` = optional