Mention that we need zlib.

--HG--
branch : HEAD
diff --git a/README b/README
index 94bde23..ed5f78b 100644
--- a/README
+++ b/README
@@ -9,6 +9,9 @@
 is in the src/ subdirectory.
 
   For generic installation instructions, see file `INSTALL'.
+Additionally, you'll need zlib (at least version 1.1.2). It comes
+with most operating systems nowadays, or you can get it at
+	http://www.gzip.org/zlib/
 
   If you make a binary distribution, please include a pointer to the
 distribution site: