Update README.md to include Tail Call proposal

Author: kateinoigakukun

README.md

@@ -68,6 +68,7 @@ and should work on the following platforms:
 |                      | [Sign-extension operators](https://github.com/WebAssembly/spec/blob/master/proposals/sign-extension-ops/Overview.md) | ✅ Implemented |
 |                      | [Non-trapping float-to-int conversions](https://github.com/WebAssembly/nontrapping-float-to-int-conversions/blob/main/proposals/nontrapping-float-to-int-conversion/Overview.md) | ✅ Implemented |
 |                      | [Memory64](https://github.com/WebAssembly/memory64/blob/main/proposals/memory64/Overview.md) | ✅ Implemented |
+|                      | [Tail call](https://github.com/WebAssembly/tail-call/blob/master/proposals/tail-call/Overview.md) | ✅ Implemented |
 |                      | [Threads and atomics](https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md) | 🚧 Parser implemented |
 | WASI | WASI Preview 1 | ✅ Implemented |