Add README for yarn modules

yarn/README.md

+# YARN DIRECTORY LAYOUT
+
+Hadoop Yarn related code are organized 
+
+Hadoop Yarn related codes are organized in separate modules with layered dependency.
+
+ * common : Common codes that will be called into by other modules.
+
+ * 2.0 / 2.2 : Codes that involve specific version of Hadoop YARN API. Depends on common module.
+
+  2.0 actually represents  0.23 and 2.0
+  2.2 actually represents 2.2 and later, until the API is break again.
+
+ * Scheduler : Implementation of various YARN Scheduler and backend. Depends on 2.0 / 2.2 modules.
+ * Assembly : For maven build to assembly all other modules in Yarn into one single jar.
+