Enhance README.md with KDL patching guidelines and documentation (#2317)

Co-authored-by: saschanaz <saschanaz@users.noreply.github.com>

Author: Bashamega

inputfiles/README.md

@@ -4,6 +4,45 @@ Some files in this directory are generated. Please do not edit them.
 This specifically includes:
 
 * `idl/*`
-* `mdn/*`
+* `mdn.json`
 
 Feel free to send a pull request with changes to any other files.
+Note It is recommended to add your patch use KDL format, continue reading to find out more.
+
+## Patches
+
+The `patches/` directory contains `.kdl` files that specify modifications ("patches") on top of the Web IDL from web specifications, using the [KDL format](https://kdl.dev/).
+These patches are applied by [`patches.ts`](../src/build/patches.ts).
+
+### When to add a patch
+
+- Add extra types to the generated types, e.g. type parameters.
+- Make types more strict, e.g. replacing a string type into a string literal type.
+- Remove features that are not widely supported by web browsers, in case it's not automatically removed.
+
+### When not to add a patch
+
+- When the type is incorrect, and that's from an upstream spec. It's recommended to file a bug in the corresponding spec, and when the fix happens, it will be applied here automatically.
+
+### How to write a patch
+
+- Try to add a new patch file when the newly desired patch is big, e.g. as long as a whole page. If it's just a few lines then it can usually go into one of the existing files.
+- Files are named per their originating web specification. https://respec.org/xref/ helps you search the specifications. If there's no existing patch with that name, you should add one even if the patch will be very small. 
+- Please add code comment about the intent for the patch, e.g. a feature is removed as it's only implemented in one browser.
+- You can largely follow the Web IDL structure but in KDL syntax:
+  - Most top level types e.g. `enum`, `interface`, or `dictionary` have the same names, but multi-word names like `interface mixin` are hyphened as `interface-mixin`.
+  - Attributes and operations are called `property` and `method` respectively to follow TypeScript convention.
+- If in doubt, feel free to file an issue or request help in [Discord dom-lib-generator channel](https://discord.gg/kRYw84uG).
+
+#### Example (`patches/touch-events.kdl`)
+
+```kdl
+interface-mixin GlobalEventHandlers {
+  // Touch event handlers are intentionally hidden in non-mobile web browsers.
+  // See w3c.github.io/touch-events#dfn-expose-legacy-touch-event-apis.
+  property ontouchcancel optional=#true
+  property ontouchend optional=#true
+  property ontouchmove optional=#true
+  property ontouchstart optional=#true
+}
+```

src/build/patches.ts

@@ -1,3 +1,8 @@
+//! This file:
+//! - Locates all KDL files in the `patches/` directory.
+//! - Parses and type-checks them.
+//! - Merges their contents and applies structural transformations over the main IDL dataset.
+
 import { parse, type Value, type Node, Document } from "kdljs";
 import type {
   Enum,