add note of how to support table with more than 22 fields

Author: wangfei <wangfei1@huawei.com>

Closes #564 from scwf/patch-6 and squashes the following commits:

a331876 [wangfei] Update sql-programming-guide.md
685135b [wangfei] Update sql-programming-guide.md
10b3dc0 [wangfei] Update sql-programming-guide.md
1c40480 [wangfei] add note of how to support table with 22 fields

docs/sql-programming-guide.md

@@ -114,6 +114,8 @@ val sqlContext = new org.apache.spark.sql.SQLContext(sc)
 import sqlContext._
 
 // Define the schema using a case class.
+// Note: Case classes in Scala 2.10 can support only up to 22 fields. To work around this limit, 
+// you can use custom classes that implement the Product interface.
 case class Person(name: String, age: Int)
 
 // Create an RDD of Person objects and register it as a table.