added note comment for unused enum

This enum is not currently in use, and I've documented it accordingly. Future versions of the library will likely support bidirectional search, so this may be used in the future.

src/AwFmIndex.h

@@ -22,10 +22,10 @@
 #define AW_FM_AMINO_CARDINALITY             20
 
 
-
 enum AwFmAlphabetType{
   AwFmAlphabetAmino = 1, AwFmAlphabetNucleotide = 2};
 
+//NOTE: not currently used, but this enum is kept for future use.
 enum AwFmBwtType{
   AwFmBwtTypeBackwardOnly = 1, AwFmBwtTypeBiDirectional = 2};