Update README.md to add link for document website (#1722)

README.md

@@ -5,7 +5,7 @@ WebAssembly Micro Runtime
 
 [BA]: https://bytecodealliance.org/
 
-**[Website](https://bytecodealliance.github.io/wamr.dev)**  **[Chat](https://bytecodealliance.zulipchat.com/#narrow/stream/290350-wamr)**
+**[Guide](https://wamr.gitbook.io/)**  **[Website](https://bytecodealliance.github.io/wamr.dev)**  **[Chat](https://bytecodealliance.zulipchat.com/#narrow/stream/290350-wamr)**
 
 [Build WAMR](./doc/build_wamr.md) | [Build AOT Compiler](./README.md#build-wamrc-aot-compiler) | [Embed WAMR](./doc/embed_wamr.md) | [Export Native API](./doc/export_native_api.md) | [Build WASM Apps](./doc/build_wasm_app.md) | [Samples](./README.md#samples)