message: update mutation status in doc
diff: @@ -22,7 +22,7 @@ Feature | C++ | Java | C# | Go | Python | JS
  ------------------------------ | ------ | ------ | ------ | ------ | ------ | --------- | ------ | --- | ----
  Codegen for all basic features | Yes | Yes | Yes | Yes | Yes | Yes | Yes | WiP | WiP
  JSON parsing | Yes | No | No | No | No | No | Yes | No | No
  -Simple mutation | Yes | WIP | WIP | No | No | No | No | No | No
  +Simple mutation | Yes | Yes | Yes | Yes | No | No | No | No | No
  Reflection | Yes | No | No | No | No | No | Basic | No | No
  Buffer verifier | Yes | No | No | No | No | No | Yes | No | No
  Testing: basic | Yes | Yes | Yes | Yes | Yes | Yes | Yes | ? | ?
  