message: Add "Improve Existing Issues" notes to contribution guide
diff: @@ -14,6 +14,11 @@ When creating a new bug report do:
  - Search against existing issues to check if somebody else has already reported your problem or requested your idea
  - Fill out the issue template.
  +### Improve Existing Issues
  +- Search for [duplicate issues](https://github.com/VSCodeVim/Vim/issues?q=is%3Aissue+is%3Aopen+cursor). See which thread(s) are more mature, and recommend the duplicate be closed, or just provide links to related issues.
  +- Find [old issues](https://github.com/VSCodeVim/Vim/issues?page=25&q=is%3Aissue+is%3Aopen) and test them in the latest version of VSCodeVim. If the issue has been resolved, comment & recommend OP to close (or provide more information if not resolved).
  +- Give thumbs up / thumbs down to existing issues, to indicate your support (or not)
  +
  ## Submitting Pull Requests
  Pull requests are _awesome_.
  