Add Hunspell README (#20872)

GitOrigin-RevId: 0b02a1a03e467d0857766e70e3a3862f939c773c

services/web/frontend/js/features/source-editor/hunspell/wasm/README.md

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+# Hunspell
+
+The files in this directory are:
+
+* `hunspell.wasm`: [Hunspell](https://github.com/hunspell/hunspell) compiled to WebAssembly using Emscripten, via the [build.sh](../build.sh) script.
+* `hunspell.mjs`: a JavaScript wrapper for the WebAssembly module, generated by Emscripten.
+* `hunspell.d.ts`: manually-created types for the exports from the JavaScript module.