Document how to set default value

### Problem
I would like to set a default for some fields.

### Research
Read README, examples, issues, browsed code, no definite pointer.

[Searching the ckanext-scheming repo for "default"](https://github.com/ckan/ckanext-scheming/search?q=default) brought up https://github.com/ckan/ckanext-scheming/blob/d441d2dee72a7382a73de4185b01fae4b2d37473/ckanext/scheming/templates/scheming/package/snippets/resource_form.html#L31-L33

### Suggestions
The docs and examples could have a friendly pointer on how to set defaults, such as:

There are three ways to choose a default value.

First, if `required` is set on a choices list, the first option will be selected. (HT @jqnatividad https://github.com/ckan/ckanext-scheming/issues/265)

Second, the key `default` sets an explicit default, and works on any field. The value must be valid as per the field's datatype and validators, or exist in the choices.

```
- field_name: license_id
label: License
display_property: dcat:Dataset/dcat:distribution/dcat:Distribution/dct:license
form_snippet: license.html
default: "cc-by-sa"
```

Third, field values can be supplied through GET request parameters to "Add dataset". (HT @wardi https://github.com/ckan/ckanext-scheming/issues/258)

```
/dataset/new?notes=some_default_notes
```

Start with the README, examples, and the linked resource_form.html snippet to confirm where schema behavior is documented. Add a friendly explanation of the three default-value methods, including the provided schema and URL examples; done means a reader can find and follow the guidance.

Written by the indexing model from the issue text.