Clarified use of Hadoop version numbers in builds, following suggesti…

…on from Yevgen Yampolskiy.

README

@@ -9,7 +9,7 @@ for the first edition at http://github.com/tomwhite/hadoop-book/tree/1e, and
 for the second edition at http://github.com/tomwhite/hadoop-book/tree/2e.
 
 This version of the code has been tested with:
- * Hadoop 1.0.1/0.22.0/0.23.0
+ * Hadoop 1.0.2/0.22.0/0.23.0
  * Avro 1.5.4
  * Pig 0.9.1
  * Hive 0.8.0
@@ -27,6 +27,9 @@ Then you can build the code with:
 
 % mvn package -DskipTests -Dhadoop.version=1.0.2
 
+(Note that only the Hadoop version numbers specified in hadoop-meta/pom.xml
+can be used here, not any version of Hadoop.)
+
 You should then be able to run the examples from the book.
 
 Chapter names for "Hadoop: The Definitive Guide", Third Edition