Problem: contributors to the repo have no public recognition

[zolotokrylin]

Problem

Contributors who improve the hiring process, workflows, or templates have no
visibility in the repo. There is no way to see who is actively building the
platform.

Solution

Add a Contributors section to the README ranked by merged PRs and reviews,
auto-updated by the existing readme-update workflow.

• Goal: turn the trial repo into a viral job board #1170