Add sections

index.mld

@@ -18,11 +18,17 @@ server-reason-react is an implementation of react-dom/server and some of React's
   // <div><h1>Hello visitor</h1></div>
 ]}
 
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+{1:core Core}
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 server-reason-react provides a React and ReactDOM modules with the same interface as reason-react, including the JSX transformation via server-reason-react-ppx.
 
 - {!React} server-reason-react.react
 - {!ReactDOM} server-reason-react.reactDom
 
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+{1:extra Extra}
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 Aside from the core modules, there're a few utilities that will be handy to make your frontend code be native. Those are {!Belt}, {!Js}, [Promise], and [Webapi].
 
 All of them are implementation of the same APIs as their originals. The level of stability of those extra libraries is not as ready for production as React or ReactDOM.