Add response data to README

[bitwes]

I ended up creating a mock store which I plan to make available after I've vetted it. In doing so I created a bunch of constants and dictionaries to help create and process test responses. I'm not 100% sure if it's all correct but this information would be very helpful in the README.

# Use with queryPurchases and querySkuDetails
var SKU_TYPE = {
	INAPP = "inapp",
	SUBS = "subs"
}


# (Android) Purchase.PurchaseState
# (Godot) make_purchase_repsone -> purchase_state
var PURCHASE_STATE  = {
	PENDING  = 2,
	PURCHASED = 1,
	UNSPECIFIED_STATE = 0
}


# (Android) BillingClient.BillingResponseCode
# (Godot) purchase_error signal -> response_code
#		  This is also the response_code for all error signals.
var BILLING_RESPONSE_CODE = {
	# Billing API version is not supported for the type requested.
	BILLING_UNAVAILABLE = 3,
	# Invalid arguments provided to the API.
	DEVELOPER_ERROR = 5,
	# Fatal error during the API action.
	ERROR = 6,
	# Requested feature is not supported by Play Store on the current device.
	FEATURE_NOT_SUPPORTED = -2,
	# Failure to purchase since item is already owned.
	ITEM_ALREADY_OWNED = 7,
	# Failure to consume since item is not owned.
	ITEM_NOT_OWNED = 8,
	# Requested product is not available for purchase.
	ITEM_UNAVAILABLE = 4,
	# Success.
	OK = 0,
	# Play Store service is not connected now - potentially transient state.
	SERVICE_DISCONNECTED = -1,
	# The request has reached the maximum timeout before Google Play responds.
	SERVICE_TIMEOUT =  -3,
	# Network connection is down.
	SERVICE_UNAVAILABLE = 2,
	# User pressed back or canceled a dialog.
	USER_CANCELED = 1
}

# Array of these is sent with the sku_details_query_completed
# signal.
var SkuDetailsResponse = {
	"description":"",
	"free_trial_period":"",
	"icon_url":"",
	"introductory_price":"",
	"introductory_price_amount_micros": -1,
	"introductory_price_cycles": -1,
	"introductory_price_period": "",
	"original_price":"",
	"original_price_amount_micros": -1,
	"price":"",
	"price_amount_micros":-1,
	"price_currency_code":"",
	"sku":"",
	"subscription_period": "",
	"title":"",
	"type":""
}

# queryPurchases contains an array of these in the "purchases"
# key and an array of these is sent with the purchases_updated
# signal.
var PurchaseResponse = {
	"is_acknowledged":false,
	"is_auto_renewing":false,
	"order_id":"",
	"package_name":"",
	"purchase_state":-1, # PURCHASE_STATE
	"purchase_time":-1,
	"purchase_token":"",
	"signature":"",
	"sku":""
}


var QueryPurchaseResponse = {
	# 0 = OK, 1 = error.  response_code and debug_message will  be
	# added when an error occurs.
	"status": -1,
	"purchases":[], # array of PurchaseResponse
	"response_code":"", # only exists when status =  1
	"debug_message":"" # only exists when status =  1
}

Also, this is the best starting point I found for understanding how to use the API, which would be useful information for first timers. https://developer.android.com/google/play/billing/integrate

[Valeryn4]

We really need this!