Also add info on the tail-call proposal

Author: alexcrichton

src/attributes/codegen.md

@@ -284,6 +284,7 @@ Feature               | Implicitly Enables  | Description
 `simd128`             |                     | [WebAssembly simd proposal][simd128]
 `multivalue`          |                     | [WebAssembly multivalue proposal][multivalue]
 `reference-types`     |                     | [WebAssembly reference-types proposal][reference-types]
+`tail-call`           |                     | [WebAssembly tail-call proposal][tail-call]
 
 [bulk-memory]: https://github.com/WebAssembly/bulk-memory-operations
 [extended-const]: https://github.com/WebAssembly/extended-const
@@ -293,6 +294,7 @@ Feature               | Implicitly Enables  | Description
 [sign-ext]: https://github.com/WebAssembly/sign-extension-ops
 [simd128]: https://github.com/webassembly/simd
 [reference-types]: https://github.com/webassembly/reference-types
+[tail-call]: https://github.com/webassembly/tail-call
 [multivalue]: https://github.com/webassembly/multi-value
 
 ### Additional information