fix: update README to reflect v3.1-RC2 version and add new index.d.ts file

Author: DanielKerger

models/typescript/README.md

@@ -16,7 +16,7 @@ npm install gbfs-typescript-types --save-dev
 ## Versions
 - v2.3
 - v3.0
-- v3.1-RC
+- v3.1-RC2
 
 ## Example Code
 ```typescript

models/typescript/v3.1-RC2/index.d.ts

@@ -0,0 +1,15 @@
+// Exports auto generated files
+
+export { GbfsVersions } from './gbfs_versions';
+export { Gbfs } from './gbfs';
+export { GeofencingZones } from './geofencing_zones';
+export { Manifest } from './manifest';
+export { StationInformation } from './station_information';
+export { StationStatus } from './station_status';
+export { SystemAlerts } from './system_alerts';
+export { SystemInformation } from './system_information';
+export { SystemPricingPlans } from './system_pricing_plans';
+export { SystemRegions } from './system_regions';
+export { VehicleStatus } from './vehicle_status';
+export { VehicleTypes } from './vehicle_types';
+export { VehicleAvailability } from './vehicle_availability';