Add missing script to readme example
diff --git a/README.md b/README.md
index 9d9a956..9846eab 100644
--- a/README.md
+++ b/README.md
@@ -98,8 +98,9 @@
```
FROM jenkins
-COPY plugins /usr/share/jenkins/ref/plugins
+COPY plugins.txt /usr/share/jenkins/ref/
COPY custom.groovy /usr/share/jenkins/ref/init.groovy.d/custom.groovy
+RUN /usr/local/bin/plugins.sh /usr/share/jenkins/ref/plugins.txt
```
When jenkins container starts, it will check JENKINS_HOME has this reference content, and copy them there if required. It will not override such files, so if you upgraded some plugins from UI they won't be reverted on next start.