maintenance contacts/contractors must contains at least one

[valeriocomo]

maintenance section is defined as stated at the following link.

As defined by the standard, a publiccode.yml is compliant even if a list of contacts or a list of contractors is defined but empty.

It sounds weird and I started to search for old discussion about maintenance section.
I found out this issue (#30).

I suggest to declare explicitly mandatory that at least one item must be present in these lists.