document plugin installation in rpm/deb environments

Currently there's no mention what is the right command to install a plugin when logstash is deploying through the rpm or debian packages.

Since logstash is installed using the logstash user, installing a plugin should be done using:

7.12 and before:

`sudo -u logstash /usr/share/logstash/bin/logstash-plugin install logstash-plugin-name`

7.13 and after:

`sudo /usr/share/logstash/bin/logstash-plugin install Logstash-plugin-name`

No documentation file or test is named. Start by locating the package-installation documentation for Logstash, then document the version-specific commands shown in the issue for installing plugins; done when users deploying RPM/Debian packages can find both commands and their applicable versions.

Written by the indexing model from the issue text.