fix: adding reference to new SDK location (#389)

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<!-- CURSOR_SUMMARY -->
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> [!NOTE]
> **Low Risk**
> Low risk documentation-only change that adds guidance for migrating to
the renamed package and new repository/issue tracker.
> 
> **Overview**
> Adds a prominent notice at the top of `README.md` stating that
`launchdarkly-react-client-sdk` has been renamed to
`@launchdarkly/react-sdk`, and links to the new repository location and
issue tracker for future updates/support.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
b4fb961. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
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Author: joker23

README.md

@@ -1,3 +1,10 @@
+# Use [@launchdarkly/react-sdk](https://www.npmjs.com/package/@launchdarkly/react-sdk) instead of this package.
+
+The `launchdarkly-react-client-sdk` project has been renamed `@launchdarkly/react-sdk` and all future releases will be made from the
+[new repository](https://github.com/launchdarkly/js-core/tree/main/packages/sdk/react). Please consider upgrading and filing issues
+and requests in that repository's [issue tracker](https://github.com/launchdarkly/js-core/issues?q=is%3Aissue+is%3Aopen+label%3A%22package%3A+sdk%2Freact%22+sort%3Aupdated-desc).
+# ☝️☝️☝️☝️☝️☝️
+
 # LaunchDarkly Client-side SDK for React
 
 [![Circle CI](https://circleci.com/gh/launchdarkly/react-client-sdk/tree/main.svg?style=svg)](https://circleci.com/gh/launchdarkly/react-client-sdk/tree/main)