MINOR: [R][Docs] Add note about RSPM binary installation

[eitsupi]

No description provided.

[pitrou]

@assignUser Can you review this PR?

[assignUser]

I tested the user agent vs. the script rstudio provides ✔️
Just one small comment :)

[assignUser]

👍 looks good to me @pitrou

[ursabot]

Benchmark runs are scheduled for baseline = 1a00075 and contender = 55022ce. 55022ce is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Failed ⬇️0.7% ⬆️0.04%] test-mac-arm
[Finished ⬇️0.0% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.67% ⬆️0.12%] ursa-thinkcentre-m75q
Buildkite builds:
[Finished] 55022ced ec2-t3-xlarge-us-east-2
[Failed] 55022ced test-mac-arm
[Finished] 55022ced ursa-i9-9960x
[Finished] 55022ced ursa-thinkcentre-m75q
[Finished] 1a000751 ec2-t3-xlarge-us-east-2
[Finished] 1a000751 test-mac-arm
[Finished] 1a000751 ursa-i9-9960x
[Finished] 1a000751 ursa-thinkcentre-m75q
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