How does importing a content type work?
To add a field to custom content type you need 


field.field.node.content-type.field-name.yml
field.storage.node.field-name.yml


The easiest way for you is create the content type and fields via UI and then just simply export it in admin/config/development/configuration/single/export (you have to install the config core module). Then remove  uuid and you are done.

While exporting, export following configuration types..


Content Type
Entity form display
Entity view display
Field
Field Storage


Hope this will help you.