README update: added "for Big Data".

README.md

 # Apache Spark
 
-Spark is a fast and general cluster computing system. It provides
+Spark is a fast and general cluster computing system for Big Data. It provides
 high-level APIs in Scala, Java, and Python, and an optimized engine that
 supports general computation graphs for data analysis. It also supports a
 rich set of higher-level tools including Spark SQL for SQL and structured