indicate to Restart Elasticsearch whenever plugin is installed

[TomonoriSoejima]

When you install a plugin, sometimes you take it that it is enabled upon installation, but in reality, you need to restart Elasticsearch to activate it, as it is instructed in the guide

it would be nice if we can add some text to guide users to restart Elasticsearch.

$ ./bin/elasticsearch-plugin install mapper-size
-> Downloading mapper-size from elastic
[=================================================] 100%   
-> Installed mapper-size
$ 

[elasticmachine]

Pinging @elastic/es-core-infra

[williamrandolph]

It should be easy to add a message after successfully installing a plugin along the lines of After installation, the node must be restarted before the plugin becomes visible.

[qlchan24]

Hi I can try to implement this!