docs: restore the contributor pictures

They were not removed -- the markup has not changed since 2019. Their
source broke: opencollective.com/gpujs/contributors.svg now answers HTTP
500 with `Cannot query field "githubContributors" on type
"CollectiveInterface"`, an error from OpenCollective's own image
generator against their own API. It fails for every collective, not just
this one (webpack's returns the same), so it is not something this repo
can fix or wait out.

The image now comes from contrib.rocks, which renders the same 52
contributors from the GitHub graph the link already pointed at. The
backers and sponsor images are untouched -- those endpoints still work.

Every image URL in the README verified 200.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RUTVDFaHav3uAdN3XfZLyx

Author: fuzzie360

README.md

@@ -1435,7 +1435,11 @@ Create issues [here](https://github.com/gpujs/gpu.js/issues) and follow the temp
 ### Contributors
 
 This project exists thanks to all the people who contribute. [[Contribute](CONTRIBUTING.md)].
-<a href="https://github.com/gpujs/gpu.js/graphs/contributors"><img src="https://opencollective.com/gpujs/contributors.svg?width=890&button=false" /></a>
+<!-- Was opencollective.com/gpujs/contributors.svg, which now returns HTTP 500
+     for every collective — their image generator queries a GraphQL field
+     ("githubContributors") that their own API no longer has. The backers and
+     sponsor images below still work, so only this one moved. -->
+<a href="https://github.com/gpujs/gpu.js/graphs/contributors"><img src="https://contrib.rocks/image?repo=gpujs/gpu.js" /></a>
 
 
 ### Backers