Docs need some guidance on writing custom fields

[lindyhopchris]

There's nothing in the fields chapter of the docs about how to write your own fields. This probably needs documenting.

Raised because someone asked on Slack about supporting spatial fields; they're using this package:
https://github.com/grimzy/laravel-mysql-spatial

There's no where in the docs to point them to how to write their own field.

[tdwesten]

This is what works for now;

<?php

namespace App\JsonApi\V1\Fields;

use LaravelJsonApi\Eloquent\Fields\Attribute;
use Illuminate\Database\Eloquent\Model;
use Grimzy\LaravelMysqlSpatial\Types\Point as GeoPoint;


class Point extends Attribute
{
    /**
     * Create a string attribute.
     *
     * @param  string      $fieldName
     * @param  string|null $column
     * @return Str
     */
    public static function make(string $fieldName, string $column = null): self
    {
        return new self($fieldName, $column);
    }

    /**
     *  Assert that the attribute JSON value is as expected.
     *
     * @param  [type] $value
     * @return void
     */
    protected function assertValue($value): void
    {
        if (!is_null($value) && !is_string($value)) {
            throw new \UnexpectedValueException(
                sprintf(
                    'Expecting the value of attribute %s to be a GeoPoint.',
                    $this->name()
                )
            );
        }
    }

    public function fill(Model $model, $value, array $validatedData): void
    {
        $column = $this->column() ?: $model->getRouteKeyName();
        $coordinates = $value['coordinates'];

        $model->{$column} = new GeoPoint($coordinates[0], $coordinates[1]);
    }
}