Document CREATE plugin_settings is required before running GET plugin_settings

Hi,

It is not quite clear that when a plugin is loaded into the GoCD server that a base plugin settings object is not created.

Running a [get plugin settings](https://api.gocd.org/17.11.0/#get-plugin-settings) api call will result in a 404, until a user creates plugin settings in the UI, or runs [create plugin settings](https://api.gocd.org/17.11.0/#create-plugin-settings).

This led to some confusion and if clarified will make it easier for developers to evaluate the service when considering automated plugin settings creation.

Thanks!