CDAP-527: update README file

changes in documentation related to https://github.com/cdap-guides/cdap-spark-guide/pull/14/
This does need some adjustment now.
I notice that the the PageRankProgram has both the stream and the dataset it uses hard-coded. Is that a good model to demonstrate? Shouldn't they be passed in from the PageRankApp, as it creates them?

Here's what I would suggest, as far as the doc goes:
-1. Change the configure method (and code!) so that the order matches the order of discussion:

```
public void configure() {
  setName("PageRankApplication");
  addStream(new Stream("backlinkURLStream"));
  addSpark(new PageRankSpark());
  addService("PageRankService", new PageRankHandler());
  ...
```

-2. Then the order of the commentary would make sense: Stream, Spark, Service, ...

-3. Add this as new text as line #93:

In this example, we use a Stream to supply backlinks data; the Spark program that computes the PageRank of the web pages reads directly from the Stream. ``backlinkURLStream`` receives backlinks information in the form of two URLs separated by whitespace: ...

-4. Change (old line #100) to read:

We’ll use Scala to write the Spark program (for an example of using Java, ...

-5. Change the diagram. Remove the backLinks dataset, as it's gone.
I think the Stream should be added; see http://docs.cask.co/tutorial/current/en/tutorial2.html and its diagram for a possible example. 
I think the stream should show with the data flowing through the Spark program to the pageRanks Dataset, and then to the PageRank Service which is proving the results...

-6. Line 26 says "Use a Dataset to store the input data; and..." 
This is no longer the case. You read directly from a Stream. The dataset is no longer the input of the Spark program. Change lines 17 and onwards to be:

```
- Use a
   [Stream](http://docs.cdap.io/cdap/current/en/developers-manual/building-blocks/streams.html)
   as the source of backlinks data;
 - Build a CDAP
   [Spark](http://docs.cdap.io/cdap/current/en/developers-manual/building-blocks/spark-jobs.html)
   program that reads directly from the Stream and computes the PageRank of the web pages;
 - Use a
   [Dataset](http://docs.cdap.io/cdap/current/en/developers-manual/building-blocks/datasets/index.html)
   to store the output of the Spark program; and
 - Build a
   [Service](http://docs.cdap.io/cdap/current/en/developers-manual/building-blocks/services.html)
   to serve the PageRank computation results over HTTP.
````