Add link to website and fixed link in README.md
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# Apache NiFi [![Build Status](https://travis-ci.org/apache/nifi.svg?branch=master)](https://travis-ci.org/apache/nifi)
-Apache NiFi is an easy to use, powerful, and reliable system to process and distribute data.
+[Apache NiFi](https://nifi.apache.org/) is an easy to use, powerful, and
+reliable system to process and distribute data.
## Table of Contents
