note in CONTRIBUTING that _everyone_ is welcomed and encouraged to review PRs (#61604)

I have seen, from time to time, comments especially on non-github fora
like discourse like "XXX pr looks good and I want it, but I'm not a
reviewer," but then that user doesn't comment the same on the actual PR.

this should hopefully lower, slightly, the intimidation factor to
provide what can be useful reviews on PRs even from first time
contributors or non-committers.