docs: Add a note on full CI run in contributing guide

[terrytangyuan]

This is based on my observation through the past PRs. Feel free to correct if I have missed anything or misunderstood the process.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.

💬 Join our developer Slack at https://slack.vllm.ai to discuss your PR in #pr-reviews, coordinate on features in #feat- channels, or join special interest groups in #sig- channels.

Just a reminder: PRs would not trigger full CI run by default. Instead, it would only run fastcheck CI which starts running only a small and essential subset of CI tests to quickly catch errors. You can run other CI tests on top of those by going to your fastcheck build on Buildkite UI (linked in the PR checks section) and unblock them. If you do not have permission to unblock, ping simon-mo or khluu to add you in our Buildkite org.

Once the PR is approved and ready to go, your PR reviewer(s) can run CI to test the changes comprehensively before merging.

To run CI, PR reviewers can either: Add ready label to the PR or enable auto-merge.

🚀

[DarkLight1337]

I think this is unnecessary since it's also mentioned by the bot.

[terrytangyuan]

Oh thanks for pointing that out. I did not notice it. However, I think this is still nice to have this is the doc contributors would read even before submitting the PR. It's also in the spirit of making the process transparent as claimed in the doc:

We would like to make the review process transparent and efficient and make sure no contributor feels confused or frustrated

[DarkLight1337]

Sure!