added typings for typescript

index.d.ts

@@ -0,0 +1,18 @@
+declare module "srt-webvtt" {
+  class WebVTTConverter {
+    constructor(resource: Blob);
+
+    public blobToBuffer(): Promise<Uint8Array | string>;
+    public static blobToString(
+      blob: Blob,
+      success: (result: string) => void,
+      fail: () => void
+    ): void;
+    public static toVTT(utf8str: string): string;
+    public static toTypedArray(str: string): Uint8Array;
+    public getURL(): Promise<string>;
+    public release(): void;
+  }
+
+  export default WebVTTConverter;
+}