[MINOR] Add GitHub Action build status badge to the README

[williamhyun]

What changes were proposed in this pull request?

This PR aims to add GitHub Action build status badge to README.md.

Why are the changes needed?

This will improve the visibility of the build status.

• https://github.com/williamhyun/spark/tree/badge#apache-spark

Does this PR introduce any user-facing change?

No.

How was this patch tested?

N/A

[SparkQA]

Kubernetes integration test unable to build dist.

exiting with code: 1
URL: https://amplab.cs.berkeley.edu/jenkins/job/SparkPullRequestBuilder-K8s/44467/

[SparkQA]

Test build #139940 has finished for PR 32954 at commit 95b6b04.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[dongjoon-hyun]

Thank you, @williamhyun . Merged to master.