Adds more warnings to README.

README.md

@@ -29,3 +29,23 @@ If you also install either the [Unicode](http://github.com/blackwinter/unicode)
 **or** [Active Support](http://github.com/rails/rails) gems, Unidecoder will
 also perform Unicode normalization before attempting to transliterate strings
 to ASCII.
+
+## Warnings
+
+While this is a neat trick, in practice many transliterations end up being
+fairly useless. For example, all Chinese characters are transliterated to
+Mandarin Chinese. Since Japanese uses Chinese characters writing, but
+pronounces them differently from Mandarin, this makes the transliteration of
+Japanese with this library useless.
+
+Some languages, like Russian, would most correctly transliterate some letters
+based on context, rather than a 1-1 mapping with ASCII. This library does not
+do that.
+
+Other languages, like Hebrew and Arabic, don't write vowels, but assume them
+from context, so the ASCII representation of these langages given by this
+library will look fairly ugly to native speakers.
+
+Basically, your milage may vary. I don't speak every language used by this
+library, so there are certain to be limitations and errors. Your feedback is
+most appreciated!