Point out validate.sh more prominently in CONTRIBUTING.md

[9999years]

The CONTRIBUTING.md makes it sound really hard to run tests locally, but actually validate.sh works fine!

validate.sh still needs work (see the list in #10320) for example, but it's a great starting point.

[ffaf1]

Pointing that validate.sh runs everything locally is fine, but it is far from a tl;dr, isn't it?

Adding a sentence or a paragraph feels better. Something like “You can also run all tests locally using validate.sh”.

[ulysses4ever]

to Francesco's point: I think it's good enough. But if someone feels strong about it, they should feel free to "request changes".

[geekosaur]

It might be worth pointing out that, especially with the njobs change, running validate locally can have a pretty significant impact on a personal machine. (Admittedly, not as much so as building ghc….)

[ffaf1]

Yes, my suggestion is to:

• add after “Running tests locally.”: “./validate.sh` runs all the test suites.”
• add “There are two ways to run tests, using GitHub Actions and running them locally” as first paragraph of “Running tests” heading.

This way we should be clear and immediate enough for all use cases.