diff --git a/README.txt b/README.txt
index d470731..4a70dfe 100644
--- a/README.txt
+++ b/README.txt
@@ -18,4 +18,19 @@ They are registered as named vocabularies, so you can do::
 
 The underlying source (see source.py) implements the IQuerySource interface
 from z3c.formwidget.query. This means that it can be used for a query-select
-widget, including the one in plone.formwidget.autocomplete. 
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+widget, including the one in plone.formwidget.autocomplete. 
+
+A note about unicode
+--------------------
+
+The source attempts to make it safe to do a __contains__ check, a getTerm()
+lookup, and searches using unicode strings. This is somewhat constrained by
+the underlying plugins. In particular, the standard ZODBGroups plugin is
+incapable of searching for groups with unicode titles or ids, and returns
+a list of *all* groups if passed a unicode string. As such, the source
+forces all unicode strings used to search for groups to UTF-8 (searching for
+users is unaffected).
+
+Also, remember that tokens should be 7-bit ASCII strings. getTermByToken() is
+forgiving in that it silently encodes a unicode string to utf-8, but really
+you should only pass unicode to this method.
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