Document CREATE plugin_settings is required before running GET plugin_settings

[gregorskii]

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 api call will result in a 404, until a user creates plugin settings in the UI, or runs 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!

[arvindsv]

@gregorskii - That makes sense. Would you mind submiting a PR to this file, maybe adding a note?