Add comment to buffer layout types

Author: guibescos

src/anchor/types/buffer-layout/index.d.ts

@@ -1,3 +1,7 @@
+// Borrowed from coral-xyz/anchor
+//
+// https://github.com/coral-xyz/anchor/blob/master/ts/packages/anchor/types/buffer-layout/index.d.ts
+
 declare module 'buffer-layout' {
   // TODO: remove `any`.
   export class Layout<T = any> {