Added TypeScript Typings

Not sure if you are interested in supporting Typescript users, but I did this while using your library for myself and thought I would share. Thanks!

index.d.ts

@@ -0,0 +1,14 @@
+import * as React from "react";
+import * as ReactNative from "react-native";
+
+declare module "react-native-linear-gradient" {
+
+    interface LinearGradientProps extends ReactNative.ViewProperties {
+        colors: string[],
+        start?: { x: number, y: number },
+        end?: { x: number, y: number },
+        locations?: number[]
+    }
+
+    export default class LinearGradient extends React.Component<LinearGradientProps, any> { }
+}