add nightly release version to readme (#11780)

README.md

@@ -4,6 +4,7 @@
 <br />
 <p align="center">
   <a href="https://github.com/netdata/netdata/releases/latest"><img src="https://img.shields.io/github/release/netdata/netdata.svg" alt="Latest release"></a>
+  <a href="https://github.com/netdata/netdata/releases/latest"><img src="https://img.shields.io/badge/dynamic/xml?url=https://storage.googleapis.com/netdata-nightlies/latest-version.txt&label=nightly%20release&query=/text()" alt="Nightly release"></a>
   <a href="https://travis-ci.com/netdata/netdata"><img src="https://travis-ci.com/netdata/netdata.svg?branch=master" alt="Build status"></a>
   <a href="https://bestpractices.coreinfrastructure.org/projects/2231"><img src="https://bestpractices.coreinfrastructure.org/projects/2231/badge" alt="CII Best Practices"></a>
   <a href="https://www.gnu.org/licenses/gpl-3.0"><img src="https://img.shields.io/badge/License-GPL%20v3%2B-blue.svg" alt="License: GPL v3+"></a>