Add notice to isContract helper on constract constructors

contracts/AddressUtils.sol

@@ -7,6 +7,8 @@ library AddressUtils {
 
   /**
    * Returns whether there is code in the target address
+   * @dev This function will return false if invoked during the constructor of a contract,
+   *  as the code is not actually created until after the constructor finishes.
    * @param addr address address to check
    * @return whether there is code in the target address
    */
@@ -16,4 +18,4 @@ library AddressUtils {
     return size > 0;
   }
 
-}
+}