add typings for GithubCorner component

I use this in a TypeScript project of mine and needed to create typings for it. In order to make it easier for other to use this component I added them here.

src/lib/GithubCorner.d.ts

@@ -0,0 +1,15 @@
+import * as React from 'react';
+
+interface GithubCornerProps {
+    href?: string;
+    size?: number | string;
+    direction?: string;
+    octoColor?: string;
+    bannerColor?: string;
+    ariaLabel?: string;
+    className?: string;
+    svgStyle?: CSSProperties;
+}
+
+export default class GithubCorner extends React.Component<GithubCornerProps, any> {
+}