add doc for PR status checks [skip ci] (#3031)

* add doc for PR status checks

Signed-off-by: Peixin Li <pxli@nyu.edu>

* address comments

* wrap line

CONTRIBUTING.md

@@ -187,5 +187,22 @@ You can confirm that the update actually has happened by either inspecting its e
 `git diff` first or simply reexecuting `git commit` right away. The second time no file
 modification should be triggered by the copyright year update hook and the commit should succeed.
 
+### Pull request status checks
+A pull request should pass all status checks before merged.
+#### signoff check
+Please follow the steps in the [Sign your work](#sign-your-work) section,
+and make sure at least one commit in your pull request get signed-off.
+#### blossom-ci
+The check runs on NVIDIA self-hosted runner, a [project committer](.github/workflows/blossom-ci.yml#L36) can
+manually trigger it by commenting `build`. It includes following steps,
+1. Mergeable check
+2. Blackduck vulnerability scan
+3. Fetch merged code (merge the pull request HEAD into BASE branch, e.g. fea-001 into branch-x)
+4. Run `mvn verify` and unit tests for multiple Spark versions in parallel. 
+Ref: [spark-premerge-build.sh](jenkins/spark-premerge-build.sh)
+
+If it fails, you can click the `Details` link of this check, and go to `Post processing -> Start post processing` to
+find the uploaded log.
+
 ## Attribution
 Portions adopted from https://github.com/rapidsai/cudf/blob/main/CONTRIBUTING.md, https://github.com/NVIDIA/nvidia-docker/blob/main/CONTRIBUTING.md, and https://github.com/NVIDIA/DALI/blob/main/CONTRIBUTING.md