[compiler] Add readme for babel plugin

Adds a simple readme for the babel plugin with links to the docs.

ghstack-source-id: 4dac0b2a4b2e2114e369dd8f63eea55909f668c1
Pull Request resolved: #29084

compiler/packages/babel-plugin-react-compiler/README.md

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+# babel-plugin-react-compiler
+
+React Compiler is a compiler that optimizes React applications, ensuring that only the minimal parts of components and hooks will re-render when state changes. The compiler also validates that components and hooks follow the Rules of React.
+
+This package contains the React Compiler Babel plugin use in projects that make use of Babel. You can find instructions for using this plugin here: https://react.dev/learn/react-compiler