[SPARK-16557][SQL] Remove stale doc in sql/README.md

## What changes were proposed in this pull request?
Most of the documentation in https://github.com/apache/spark/blob/master/sql/README.md is stale. It would be useful to keep the list of projects to explain what's going on, and everything else should be removed.

## How was this patch tested?
N/A

Author: Reynold Xin <rxin@databricks.com>

Closes #14211 from rxin/SPARK-16557.