add pattern table and pointer

main.usm

@@ -367,6 +367,9 @@ BRK
 ( pattern format - each note has 3 bytes. Pitch, instrument, volume. patterns are 0x10 long.
   an ff in the note column of the pattern will kill the existing note without starting a new one (this is the tracker "box" note) )
 
+@data ( start of data )
+@pattern_table :pattern $fd ( pattern table. Contains the addresses of patterns )
+@pattern_pointer :pattern/end ( pattern pointer. Points to where the next pattern will be written in memory )
 @pattern [
 	48 12 66
 	49 34 66
@@ -383,5 +386,5 @@ BRK
 	54 00 66 
 	ff 00 66
 	56 00 ff
-	57 00 66
-]
+	57 00 66 &end
+]