message: docs: add and update README badges
diff: @@ -19,13 +19,15 @@ under the License.
  Superset
  =========
  +[![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
  +![GitHub release (latest SemVer)](https://img.shields.io/github/v/release/apache/incubator-superset)
  [![Build Status](https://travis-ci.org/apache/incubator-superset.svg?branch=master)](https://travis-ci.org/apache/incubator-superset)
  [![PyPI version](https://badge.fury.io/py/apache-superset.svg)](https://badge.fury.io/py/apache-superset)
  [![Coverage Status](https://codecov.io/github/apache/incubator-superset/coverage.svg?branch=master)](https://codecov.io/github/apache/incubator-superset)
  [![PyPI](https://img.shields.io/pypi/pyversions/apache-superset.svg?maxAge=2592000)](https://pypi.python.org/pypi/apache-superset)
  -[![Get on Slack](https://img.shields.io/badge/slack-join-orange.svg)](https://join.slack.com/t/apache-superset/shared_invite/enQtNDMxMDY5NjM4MDU0LWJmOTcxYjlhZTRhYmEyYTMzOWYxOWEwMjcwZDZiNWRiNDY2NDUwNzcwMDFhNzE1ZmMxZTZlZWY0ZTQ2MzMyNTU)
  +[![Get on Slack](https://img.shields.io/badge/slack-join-orange.svg)](https://join.slack.com/t/apache-superset/shared_invite/zt-g8lpruog-HeqpgYrwdfrD5OYhlU7hPQ)
  [![Documentation](https://img.shields.io/badge/docs-apache.org-blue.svg)](https://superset.incubator.apache.org)
  -[![dependencies Status](https://david-dm.org/apache/incubator-superset/status.svg?path=superset-frontend)](https://david-dm.org/apache/incubator-superset?path=superset-frontend)
  +[![Dependencies Status](https://david-dm.org/apache/incubator-superset/status.svg?path=superset-frontend)](https://david-dm.org/apache/incubator-superset?path=superset-frontend)
  <img
  src="https://cloud.githubusercontent.com/assets/130878/20946612/49a8a25c-bbc0-11e6-8314-10bef902af51.png"
  