updated mapformat.txt a bit

doc/mapformat.txt

@@ -8,6 +8,7 @@ generator. If they are not found or invalid, the generator will currently
 behave quite strangely.
 
 The MapBlock file format (sectors2/XXX/ZZZ/YYYY):
+-------------------------------------------------
 
 NOTE: Byte order is MSB first.
 
@@ -71,5 +72,18 @@ u32 timestamp
 - 0xffffffff = invalid/unknown timestamp, nothing will be done with the time
                difference when loaded (recommended)
 
+Node metadata format:
+---------------------
+
+Sign metadata:
+  u16 string_len
+  u8[string_len] string
+
+Furnace metadata:
+  TBD
+
+Chest metadata:
+  TBD
+
 // END