Add readme for extended const (#4471)

Author: kylo5aby

README.md

@@ -48,6 +48,7 @@ WebAssembly Micro Runtime (WAMR) is a lightweight standalone WebAssembly (Wasm)
 - [Reference Types](https://github.com/WebAssembly/reference-types), ref to [document](doc/ref_types.md) and [sample](samples/ref-types)
 - [Bulk memory operations](https://github.com/WebAssembly/bulk-memory-operations), [Shared memory](https://github.com/WebAssembly/threads/blob/main/proposals/threads/Overview.md#shared-linear-memory), [Memory64](https://github.com/WebAssembly/memory64)
 - [Tail-call](https://github.com/WebAssembly/tail-call), [Garbage Collection](https://github.com/WebAssembly/gc), [Exception Handling](https://github.com/WebAssembly/exception-handling)
+- [Extended Constant Expressions](https://github.com/WebAssembly/extended-const)
 
 ### Supported architectures and platforms
 The WAMR VMcore supports the following architectures: