Note in the README that this fork adds Anbernic stock OS support

This is a fork-only change; the upstream README is left untouched below
the banner, and the anbernic-support branch that PR rommapp#268 proposes does
not carry it.

Author: Robilol

README.md

@@ -1,4 +1,20 @@
 <!-- trunk-ignore-all(markdownlint/MD033) -->
+> [!NOTE]
+> **This is a fork of [rommapp/grout](https://github.com/rommapp/grout) that adds support for the Anbernic stock OS.**
+>
+> The stock OS is the original Anbernic firmware that ships preinstalled on the Allwinner H700
+> handhelds (RG35XX Plus/H/SP/2024, RG34XX, RG40XX, RG28XX, CubeXX) — no custom firmware required.
+>
+> - Work lives on the [`anbernic-support`](https://github.com/Robilol/grout/tree/anbernic-support) branch
+> - Proposed upstream as [rommapp/grout#268](https://github.com/rommapp/grout/pull/268)
+> - Platform folders, save paths, BIOS directory and button mapping were derived from the
+>   `RG35XX+P-EN-64G-V1.2.4-260522` firmware image and verified on a real RG35XX Plus.
+>   See [`docs/platforms/anbernic.md`](https://github.com/Robilol/grout/blob/anbernic-support/docs/platforms/anbernic.md).
+>
+> Everything below is the upstream README, unchanged.
+
+---
+
 <!-- trunk-ignore(markdownlint/MD041) -->
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