README.md: Add badge about docker

Signed-off-by: Ce Gao <ce.gao@outlook.com>

Author: gaocegege

readme.md

@@ -6,6 +6,7 @@
 <h4 align="center">Processing.R allows the users to write Processing sketches in R.</h4>
 
 [![Build Status](https://travis-ci.org/gaocegege/Processing.R.svg?branch=master)](https://travis-ci.org/gaocegege/Processing.R)
+[![](https://img.shields.io/badge/docker-supported-blue.svg)](https://quay.io/repository/gaocegege/processing.r)
 [![Docker Repository on Quay](https://quay.io/repository/gaocegege/processing.r/status "Docker Repository on Quay")](https://quay.io/repository/gaocegege/processing.r)
 [![Codacy Badge](https://api.codacy.com/project/badge/Grade/33ebc49f6f764fffb7ea7bf617edf902)](https://www.codacy.com/app/gaocegege/Processing-R?utm_source=github.com&amp;utm_medium=referral&amp;utm_content=gaocegege/Processing.R&amp;utm_campaign=Badge_Grade)
 [![Gitter](https://badges.gitter.im/gaocegege/Processing.R.svg)](https://gitter.im/gaocegege/Processing.R?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)