Policy for contributed and experimental formatting queries

[Xophmeister]

Topiary is happy to accept contributed formatting queries and, as of writing, we have merged several into main. However, while formatting queries remain direct components of the project -- rather than external, decoupled resources -- we should define a policy for contributions as a quality assurance step.

This should cover:

• Crediting of authors of formatting queries
• Support expectations of contributed queries
• The bar to pass for contribution (e.g., some threshold percentage of the grammar covered for it to be accepted, or some lower threshold for it to be considered "experimental", etc.)
• etc.