[minor] add Coverage Status in readme

[Ted-Jiang]

Which issue does this PR close?

Add code Coverage tag 😊

Rationale for this change

What changes are included in this PR?

Are there any user-facing changes?

[Ted-Jiang]

@alamb After add code Coverage forget add this tag 😂
Seems higher than arrow-rs

[ursabot]

Benchmark runs are scheduled for baseline = 4838564 and contender = 1f67876. 1f67876 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ec2-t3-xlarge-us-east-2] ec2-t3-xlarge-us-east-2
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on test-mac-arm] test-mac-arm
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-i9-9960x] ursa-i9-9960x
[Skipped ⚠️ Benchmarking of arrow-datafusion-commits is not supported on ursa-thinkcentre-m75q] ursa-thinkcentre-m75q
Buildkite builds:
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python, R. Runs only benchmarks with cloud = True
test-mac-arm: Supported benchmark langs: C++, Python, R
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java

[alamb]

Thanks @Ted-Jiang !

Seems higher than arrow-rs

I think at least one problem is that doctests don't count for coverage and arrow-rs has a bunch of doctests

[Ted-Jiang]

Thanks @Ted-Jiang !

Seems higher than arrow-rs

I think at least one problem is that doctests don't count for coverage and arrow-rs has a bunch of doctests

@alamb Got it! will try to add this doctests to the report!

[alamb]

Thanks @Ted-Jiang