add explanatory comment to estree.d.ts

Author: michaelfaith

types/estree.d.ts

@@ -1,5 +1,13 @@
 import { Program as EstreeProgram } from 'estree';
 
+/**
+ * This file augments the `estree` types to include a couple of types that are not built-in to `estree` that we're using.
+ * This is necessary because the `estree` types are used by ESLint, and ESLint does not natively support
+ * TypeScript types.  Since we're only using a couple of them, we can just add them here, rather than
+ * installing typescript estree types.
+ *
+ * This also adds support for the AST mutation that the ESLint does to add parent nodes.
+ */
 declare module 'estree' {
   interface BaseNode {
     parent: Node;