[SPARK-16557][SQL] Remove stale doc in sql/README.md
[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 <c9d35a384bc579d1c6c8c055f2a2b5c04e11b521@databricks.com>

Closes #14211 from rxin/SPARK-16557.

(cherry picked from commit 2e4075e2ece9574100c79558cab054485e25c2ee)
Signed-off-by: Reynold Xin <c9d35a384bc579d1c6c8c055f2a2b5c04e11b521@databricks.com>

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