Improve instructions for new contributors

CONTRIBUTING.md

@@ -6,11 +6,10 @@ and follow the [guidelines](https://jazzband.co/about/guidelines).
 
 ## Project Contribution Guidelines
 
-Here are a few additional or emphasized guidelines to follow when contributing to pip-tools:
+Here are a few additional or emphasized guidelines to follow when contributing to `pip-tools`:
 
-- Install pip-tools in development mode and its test dependencies with `pip install -e .[testing]`.
-- Check with `tox -e checkqa` to see your changes are not breaking the style conventions.
-- Always provide tests for your changes.
+- If you need to have a virtualenv outside of `tox`, it is possible to reuse its configuration to provision it as [described in the docs](https://tox.readthedocs.io/en/latest/example/devenv.html#creating-development-environments-using-the-devenv-option).
+- Always provide tests for your changes and run `tox -p all` to make sure they are passing the checks locally.
 - Give a clear one-line description in the PR (that the maintainers can add to [CHANGELOG](CHANGELOG.md) afterwards).
 - Wait for the review of at least one other contributor before merging (even if you're a Jazzband member).
 - Before merging, assign the PR to a milestone for a version to help with the release process.