Add paragraph on requesting reviews (bevyengine#13161)

# Objective

Requesting reviews is a useful tool for improving discoverability of PRs
that contributors might be interested in and capable of reviewing.

However, many Bevy org members and authors aren't aware that they can
and should request reviews.

## Solution

Actually document that this is good practice, and remind people that
it's just a nudge.

---------

Co-authored-by: Alice Cecile <[email protected]>

CONTRIBUTING.md

@@ -277,6 +277,11 @@ With the sheer volume of activity in Bevy's community, reviewing others work wit
 You don't need to be an Elder Rustacean to be useful here: anyone can catch missing tests, unclear docs, logic errors, and so on.
 If you have specific skills (e.g. advanced familiarity with `unsafe` code, rendering knowledge or web development experience) or personal experience with a problem, try to prioritize those areas to ensure we can get appropriate expertise where we need it.
 
+When you find (or make) a PR that you don't feel comfortable reviewing, but you *can* think of someone who does, consider using Github's "Request review" functionality (in the top-right of the PR screen) to bring the work to their attention.
+If they're not a Bevy Org member, you'll need to ping them in the thread directly: that's fine too!
+Almost everyone working on Bevy is a volunteer: this should be treated as a gentle nudge, rather than an assignment of work.
+Consider checking the Git history for appropriate reviewers, or ask on Discord for suggestions.
+
 Focus on giving constructive, actionable feedback that results in real improvements to code quality or end-user experience.
 If you don't understand why an approach was taken, please ask!