text: Title: Enable @pytest.mark.limit_memory without having to specify --memray
  Body: ## Feature Request
  
  **Is your feature request related to a problem? Please describe.**
  
  I tried to use `pytest.mark.limit_memory` in a urllib3 test and was confused because pytest was not finding it because I only skimmed the README and missed the `--memray` option.
  
  (Additionally I was surprised that `limit_memory` was not in the pytest-memray README at all. I only found about it on the memray README.)
  
  **Describe the solution you'd like**
  
  I'd like to be able to use `pytest.mark.limit_memory` without passing `--memray` and without showing the memray report at the end.
  
  **Describe alternatives you've considered**
  
  The alternative is the status quo. memray is incredibly useful, this is only a small quality-of-life improvement.
  
  **Teachability, Documentation, Adoption, Migration Strategy**
  
  If we had a code snipped for `limit_memory` in the docs, users could simply copy/paste it without having to find out about `--memray`.
  