Added reserved ExtraByte options bits to table. (#159)

Author: esilvia

source/04_optional_vlrs.txt

@@ -174,6 +174,8 @@ Any unused characters in the "name" or "description" fields must be set to zero.
     |           |                  | by the corresponding offset value (after  |
     |           |                  | applying the scaling).                    |
     +-----------+------------------+-------------------------------------------+
+    | 5-7       | *Reserved*       | *Reserved for ASPRS definition*           |
+    +-----------+------------------+-------------------------------------------+
 
 
 The bit mask in the "options" field specifies whether the min and max range of