Mention the problem with supplementary plains

Close #4.

Data/Text/Metrics.hs

@@ -11,6 +11,20 @@
 -- It works with strict 'Text' values and returns either 'Natural' numbers
 -- (because the metrics cannot be negative), or @'Ratio' 'Natural'@ values
 -- because returned values are rational non-negative numbers by definition.
+--
+-- The functions provided here are the fastest implementations available for
+-- use in Haskell programs. In fact the functions are implemented in C for
+-- maximal efficiency, but this leads to a minor flaw. When we work with
+-- 'Text' values in C, they are represented as UTF-16 encoded strings of
+-- two-byte values. The algorithms treat the strings as if a character
+-- corresponds to one element in such strings, which is true for almost all
+-- modern text data. However, there are characters that are represented by
+-- two adjoined elements in UTF-16: emoji, historic scripts, less used
+-- Chinese ideographs, and some more. If input 'Text' of the functions
+-- contains such characters, the functions may return slightly incorrect
+-- result. Decide for yourself if this is acceptable for your use case, but
+-- chances are you will never run into situations when the functions produce
+-- incorrect results.
 
 {-# LANGUAGE CPP                      #-}
 {-# LANGUAGE ForeignFunctionInterface #-}

README.md

@@ -12,6 +12,19 @@ It works with strict `Text` values and returns either `Natural` numbers
 (because the metrics cannot be negative), or `Ratio Natural` values because
 returned values are rational non-negative numbers by definition.
 
+The functions provided here are the fastest implementations available for
+use in Haskell programs. In fact the functions are implemented in C for
+maximal efficiency, but this leads to a minor flaw. When we work with `Text`
+values in C, they are represented as UTF-16 encoded strings of two-byte
+values. The algorithms treat the strings as if a character corresponds to
+one element in such strings, which is true for almost all modern text data.
+However, there are characters that are represented by two adjoined elements
+in UTF-16: emoji, historic scripts, less used Chinese ideographs, and some
+more. If input `Text` of the functions contains such characters, the
+functions may return slightly incorrect result. Decide for yourself if this
+is acceptable for your use case, but chances are you will never run into
+situations when the functions produce incorrect results.
+
 The current version of the package implements:
 
 * [Levenshtein distance](http://en.wikipedia.org/wiki/Levenshtein_distance)