Add guidance on how to specify a default sub-resource

[a-akimov]

Use case:

When a parent resource has a collection of sub-resources, sometimes there is a need to specify which one is the default. For example, there might be multiple shipping address, and one of them should be used by default.

Possible options here:

1. Store the default_address_id in a parent resource and validate it.
2. Store the boolean default field within each sub-resource and make an explicit /make_default action to update it.
3. Store the boolean default field within each sub-resource and use PATCH to update it.