Update Spark documentation to provide information on how to create External Table

## What changes were proposed in this pull request?
Although, currently, the saveAsTable does not provide an API to save the table as an external table from a DataFrame, we can achieve this functionality by using options on DataFrameWriter where the key for the map is the String: "path" and the value is another String which is the location of the external table itself. This can be provided before the call to saveAsTable is performed.

## How was this patch tested?
Documentation was reviewed for formatting and content after the push was performed on the branch.
![updated documentation](https://cloud.githubusercontent.com/assets/15376052/20953147/4cfcf308-bc57-11e6-807c-e21fb774a760.PNG)

Author: c-sahuja <sahuja@cloudera.com>

Closes #16185 from c-sahuja/createExternalTable.