message: Mark :sh[ell] as implemented in ROADMAP.md
diff: @@ -487,7 +487,7 @@ Since the list is too long, now we just put those already supported options here
  | Status | Command | Description |
  | ------------------ | ----------- | -------------------------------------------------------------------------- |
  -| :arrow_down: | :sh[ell] | start a shell |
  +| :white_check_mark: | :sh[ell] | start a shell |
  | :white_check_mark: | :!{command} | execute {command} with a shell |
  | :arrow_down: | K | lookup keyword under the cursor with 'keywordprg' program (default: "man") |
  