Add a README explaining JSON format.

# Property Configuration Files

Each JSON file in this folder is a property configuration file for reference

Vehicle HAL. They contain VehiclePropConfig information along with initial

value information.

## JSON schema

Each JSON file must be in a schema like the following example:

(The comment starting with "//" is for documentation only and must be removed

from the actual JSON file. The "comment" field is used for comment in the

actual JSON file and will be ignored by the parser)

```

{

    // (number) The version for the JSON schema.

    "apiVersion": 1,

    // (non-empty array of objects) The property configuration list.

    //

    // Each object is a configuration for one property.

    "properties": [

        {

            // (number/string) The ID for the property.

            // This value is defined in a string value

            // which represents a constant value, see the "JSON Number-type

            // Field Values" section for detail.

            "property": "VehicleProperty::INFO_FUEL_CAPACITY",

            // (optional, number/string) The access mode for the property.

            // If specified, this overwrite the default access mode specified in

            // VehicleProperty.aidl. Must be specified for vendor properties.

            "access": "VehiclePropertyAccess::READ",

            // (optional, number/string) The change mode for the property.

            // If specified, this overwrite the default change mode specified in

            // VehicleProperty.aidl. Must be specified for vendor properties.

            "changeMode": "VehiclePropertyChangeMode::STATIC",

            // (optional, string) The config string.

            "configString": "blahblah",

            // (optional, array of number/string) The config array.

            "configArray": [1, 2, "Constants::HVAC_ALL"],

            // (optional, object) The default value for the property.

            // If not specified, the property will be shown as unavailable

            // until its value is set.

            "defaultValue": {

                // (optional, array of int number/string) Int values.

                "int32Values": [1, 2, "Constants::HVAC_ALL"],

                // (optional, array of int number/string) Long values.

                "int64Values": [1, 2],

                // (optional, array of float number/string) Float values.

                "floatValues": [1.1, 2.2],

                // (optional, string) String value.

                "stringValue": "test"

            },

            // (optional, number/string) The minimum sample rate in HZ.

            // Only work for VehiclePropertyChangeMode::CONTINUOUS property.

            // Must be specified for continuous property.

            "minSampleRate": 1,

            // (optional, number/string) The maximum sample rate in HZ.

            // Only work for VehiclePropertyChangeMode::CONTINUOUS property.

            // Must be specified for continuous property.

            "maxSampleRate": 10,

            // (optional, array of objects) The area configs.

            "areas:" [

                {

                    // (number/string) The area ID.

                    "areaId": "Constants::DOOR_1_LEFT",

                    // (optional number/string) The minimum int value.

                    "minInt32Value": 1,

                    // (optional number/string) The maximum int value.

                    "maxInt32Value": 10,

                    // (optional number/string) The minimum long value.

                    "minInt64Value": 1,

                    // (optional number/string) The maximum long value.

                    "maxInt64Value": 10,

                    // (optional number/string) The minimum float value.

                    "minFloatValue": 1,

                    // (optional number/string) The maximum float value.

                    "maxFloatValue": 10,

                    // (optional object) The default value for this area.

                    // Uses the same format as the "defaultValue" field for

                    // property object. If specified, this overwrite the global

                    // defaultValue.

                    "defaultValue": {

                        "int32Values": [1, 2, "Constants::HVAC_ALL"],

                        "int64Values": [1, 2],

                        "floatValues": [1.1, 2.2],

                        "stringValue": "test"

                    }

                }

            ]

        }

     ]

}

```

## JSON Number-type Field Values

For number type field values, they can either be defined as a numeric number,

e.g., `{"minInt32Value": 1}` or be defined as a string which represents a

defined constant value, e.g.,

`{"property": "VehicleProperty::INFO_FUEL_CAPACITY"}`.

For constant values, they must be a string in the format of `XXX::XXX`, where

the field before `::` is the constant type, and the field after `::` is the

variable name.

We support the following constant types:

* VehiclePropertyAccess

* VehiclePropertyChangeMode

* VehicleGear

* VehicleAreaWindow

* VehicleOilLevel

* VehicleUnit

* VehicleSeatOccupancyState

* VehicleHvacFanDirection

* VehicleApPowerStateReport

* VehicleTurnSignal

* VehicleVendorPermission

* EvsServiceType

* EvsServiceState

* EvConnectorType

* VehicleProperty

* GsrComplianceRequirementType

* VehicleIgnitionState

* FuelType

* Constants

Every constant type except "Constants" corresponds to a enum defined in Vehicle

HAL interfac. E.g. "VehicleProperty" corresponds to the enums defined in

"VehicleProperty.aidl".

"Constants" type refers to the constant variables defined in the paresr.

Specifically, the "CONSTANTS_BY_NAME" map defined in "JsonConfigLoader.cpp".