MINOR: Add note about sqllogictest to contributor guide

[alamb]

Which issue does this PR close?

re #4460

Rationale for this change

make the harness added in #4395 by @mvanschellebeeck easier to find

What changes are included in this PR?

Add a pointer to these tests in the contributor guide

Are these changes tested?

N/A

Are there any user-facing changes?

[alamb]

cc @xudong963

[xudong963]

How about adding some comparison with Rust Integration Tests and encouraging adding logictestes as far as possible?

[ursabot]

Benchmark runs are scheduled for baseline = b229e0f and contender = 5d69fa3. 5d69fa3 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