Add a Note on jsonFile having separate JSON objects per line

* This commit hopes to avoid the confusion I faced when trying
  to submit a regular, valid multi-line JSON file, also see

  http://apache-spark-user-list.1001560.n3.nabble.com/Loading-JSON-Dataset-fails-with-com-fasterxml-jackson-databind-JsonMappingException-td20041.html
We need to move the file too and update the other places that reference it:

```
examples/src/main/java/org/apache/spark/examples/sql/JavaSparkSQL.java:    String path = "examples/src/main/resources/people.json";
examples/src/main/python/sql.py:    path = os.path.join(os.environ['SPARK_HOME'], "examples/src/main/resources/people.json")
```