update README

Author: michaelliao

README.md

@@ -1,3 +1,9 @@
 # compiler
 
 Compile Java code in memory using Java 7 compiler API.
+
+### Why compiler API?
+
+You can use compiler API to compile generated Java source code and load the compiled classes on-the-fly.
+
+For example, generate proxy classes using compiler API instead of CGLIB or Javassist.