Add authors/maintainers to the specification

[lpalgarvio]

Hello

We have Vendor, but Authors and/or Maintainers is missing in the specification.
Can we take a look at what's done in PHP Composer?

https://getcomposer.org/doc/04-schema.md#authors
`authors#

The authors of the package. This is an array of objects.

Each author object can have following properties:

name: The author's name. Usually their real name.
email: The author's email address.
homepage: An URL to the author's website.
role: The author's role in the project (e.g. developer or translator)

An example:

{
"authors": [
{
"name": "Nils Adermann",
"email": "naderman@naderman.de",
"homepage": "http://www.naderman.de",
"role": "Developer"
},
{
"name": "Jordi Boggiano",
"email": "j.boggiano@seld.be",
"homepage": "http://seld.be",
"role": "Developer"
}
]
}`

[lpalgarvio]

MAINTAINER is officially deprecated in dockerfiles.
It is not even included in docker-compose.yml files.

https://docs.docker.com/engine/reference/builder/#maintainer-deprecated

They point to implementation in labels.

[lpalgarvio]

This format is not supported in docker-compose.yml files:
`      - org.label-schema.authors:
        - "author1=bill"
        - "author2=john"`

ERROR: The Compose file './docker-compose.yml' is invalid because:
services.main.labels contains {"org.label-schema.authors": ["author1=bill", "author2=john"]}, which is an invalid type, it should be a string

But we can implement something like this:

org.label-schema.authors.bill.name=Bill Doe
org.label-schema.authors.bill.email=bill@example.com
org.label-schema.authors.bill.homepage=http://bill.example.com
org.label-schema.authors.bill.role=DevOps
org.label-schema.authors.john.name=John Doe
org.label-schema.authors.john.email=john@example.com
org.label-schema.authors.john.homepage=http://john.example.com
org.label-schema.authors.john.role=DevOps

[lpalgarvio]

Added pull request #25:

• org.label-schema.authors.john.name = "John Doe"
• org.label-schema.authors.john.email = "john@example.com"
• org.label-schema.authors.john.homepage = "http://john.example.com"
• org.label-schema.authors.john.role = "DevOps"

[lpalgarvio]

Similarly, vendor could be expanded to allow:

• org.label-schema.vendor.name = "ACME"
• org.label-schema.vendor.email = "support@example.com"
• org.label-schema.vendor.website = "http://example.com"

This change wouldn't break existing org.label-schema.vendor implementations.

[lpalgarvio]

#36 added for vendor