Update plugin.md

Plugins are only instantiated when the cordava app's JS code first call them.

If the plugin needs to be notified of a e.g. a hardware event or some other intent filter its onNewIntent method is never called unless onload = true.
<feature ...>
...
<param name="onload" value="true" />
</feature>
is a neded for all plugins that react to operating system events.

The issue for this is here:
https://issues.apache.org/jira/browse/CB-5419