Allow for multiple authors

Arises where a container is a composition of several containers. See Buildah issue opencontainers#591 specifically the current use case given in [comment](containers/buildah#591 (comment))

Author: taqtiqa-mark

config.md

@@ -98,7 +98,7 @@ Note: Any OPTIONAL field MAY also be set to null, which is equivalent to being a
 
 - **author** *string*, OPTIONAL
 
-  Gives the name and/or email address of the person or entity which created and is responsible for maintaining the image.
+  Gives the names and/or email addresses of the persons or entities which created and are responsible for maintaining the image. Multiple names should be comma delimted.
 
 - **architecture** *string*, REQUIRED