give credit to libbzip2

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/compress/trunk@1337442 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/README.txt b/README.txt
index 57b3a39..2e88366 100644
--- a/README.txt
+++ b/README.txt
@@ -2,6 +2,7 @@
 
 Original BZip2 classes contributed by Keiron Liddle
 <keiron@aftexsw.com>, Aftex Software to the Apache Ant project
+They are based on a port of Julian Seward's libbzip2.
 
 Original Tar classes from contributors of the Apache Ant project