docs: add enablePrivateMethods to compiler README

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: a-chabot

packages/@lwc/compiler/README.md

@@ -57,6 +57,7 @@ const { code } = transformSync(source, filename, options);
     - `enableLightningWebSecurityTransforms` (type: `boolean`, default: `false`) - The configuration to enable Lighting Web Security specific transformations.
     - `enableLwcSpread` (boolean, optional, `true` by default) - Deprecated. Ignored by compiler. `lwc:spread` is always enabled.
     - `disableSyntheticShadowSupport` (type: `boolean`, default: `false`) - Set to true if synthetic shadow DOM support is not needed, which can result in smaller/faster output.
+    - `enablePrivateMethods` (type: `boolean`, default: `false`) - Enable the private method round-trip transform. When false or omitted, private methods pass through to standard Babel handling.
     - `instrumentation` (type: `InstrumentationObject`, optional) - instrumentation object to gather metrics and non-error logs for internal use. See the `@lwc/errors` package for details on the interface.
     - `apiVersion` (type: `number`, optional) - API version to associate with the compiled module.