X.P.Unicode: Add docs about UnicodeData.txt

Having this file is required for the prompt to work, and how to obtain
it may not be immediately obvious.

Author: jpolchlo

XMonad/Prompt/Unicode.hs

@@ -61,6 +61,11 @@ and adding an appropriate keybinding, for example:
 
 >  , ((modm .|. controlMask, xK_u), unicodePrompt "/path/to/unicode-data" def)
 
+A path to a @UnicodeData.txt@ file or equivalent must be provided.  This file
+should be available through your package manager; search for @unicode-data@.
+If no package is found, one may opt to download this file directly from
+[unicode.org](http://www.unicode.org/Public/UNIDATA/UnicodeData.txt).
+
 More flexibility is given by the @mkUnicodePrompt@ function, which takes a
 command and a list of arguments to pass as its first two arguments. See
 @unicodePrompt@ for details.