Add portal downloads badge to README

This PR adds a download counter badge to the README using the [netbeans-download-badges](https://github.com/emilianbold/netbeans-download-badges) service.

Changes

Added download counter badge to README.md

Notes
The badge displays the total number of downloads for this plugin from the NetBeans Plugin Portal. The counter is provided by openbeans.org and updates automatically.

Author: emilianbold

README.md

@@ -6,6 +6,7 @@
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/7d825731274a4f4783e4203eb7cbf811)](https://www.codacy.com/manual/funfried/externalcodeformatter_for_netbeans?utm_source=github.com&utm_medium=referral&utm_content=funfried/externalcodeformatter_for_netbeans&utm_campaign=Badge_Grade)
 [![Maven Central](https://img.shields.io/maven-central/v/de.funfried.netbeans.plugins/externalcodeformatter)](https://repo1.maven.org/maven2/de/funfried/netbeans/plugins/externalcodeformatter/)
 [![GitHub All Releases](https://img.shields.io/github/downloads/funfried/externalcodeformatter_for_netbeans/total)](https://github.com/funfried/externalcodeformatter_for_netbeans/releases)
+[![Portal Downloads](https://img.shields.io/endpoint?url=https%3A%2F%2Fopenbeans.org%2Fplugin-counter%2Fapi%2F15&label=portal%20downloads)](https://plugins.netbeans.apache.org/catalogue/?id=15)
 [![GitHub issues](https://img.shields.io/github/issues/funfried/externalcodeformatter_for_netbeans)](https://github.com/funfried/externalcodeformatter_for_netbeans/issues)
 [![Eclipse Public License, Version 2.0](https://img.shields.io/badge/license-EPL%20v2.0-green)](http://funfried.github.io/externalcodeformatter_for_netbeans/licenses.html)
 [![Follow Me On Twitter](https://img.shields.io/twitter/follow/funfried84?style=social)](https://twitter.com/funfried84)