Consider allowing contributions to "linting" schemas

[handrews]

We have previously turned away quite a few suggested schema changes, particularly from @karenetheridge and @jeremyfiel, that attempted to enforce sensible but not mandatory constraints. These often involve SHOULDs or statements that something will be ignored rather than treated as an error.

We've discussed allowing additional schemas that would $ref the validation schemas and add these additional constraints. Let's make a formal decision whether to do that or not, and decide what would be allowed in such schemas so that folks who want to write them can do so with confidence.

[lornajane]

Agreed that we should keep to schemas that reflect the spec exactly, there are linting tools that can support adding additional rules if users need something more restrictive. We should update the Contributing file to reflect this.