Add Pull Request guidelines to contributor guide

[alamb]

Which issue does this PR close?

Closes #3957

Rationale for this change

Document how we should handle PRs

What changes are included in this PR?

Add Pull Request guidelines to contributor guide

Are there any user-facing changes?

docs

[avantgardnerio]

We could potentially add wording about it specifically being "days" not "business days" or >=1 approval, but LGTM.

[alamb]

Thanks everyone!

[alamb]

We could potentially add wording about it specifically being "days" not "business days" or >=1 approval, but LGTM.

I wanted to keep it simple to begin with -- let's try this wording and then refine with additional details if necessary.

[ursabot]

Benchmark runs are scheduled for baseline = 8f1ae58 and contender = 22dea55. 22dea55 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