Add README.md project activity badges
<a href="https://ignite.apache.org/"><img src="https://github.com/apache/ignite-website/blob/master/images/ignite_logo_full.svg" hspace="20"/></a>
[![Build Status](https://travis-ci.org/apache/ignite.svg?branch=master)](https://travis-ci.org/apache/ignite)
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+[![GitHub](https://img.shields.io/github/license/apache/ignite?color=blue)](https://www.apache.org/licenses/LICENSE-2.0.html)
+[![Maven Central](https://maven-badges.herokuapp.com/maven-central/org.apache.ignite/ignite-core/badge.svg)](https://search.maven.org/search?q=org.apache.ignite)
+[![GitHub release](https://img.shields.io/badge/release-download-brightgreen.svg)](https://ignite.apache.org/download.cgi)
+![GitHub commit activity](https://img.shields.io/github/commit-activity/m/apache/ignite)
+[![Twitter Follow](https://img.shields.io/twitter/follow/ApacheIgnite?style=social)](https://twitter.com/ApacheIgnite)
## What is Apache Ignite?
