docs(r-apt): Added case-sensitivity of apt packages warning

[mthomas-ketchbrook]

Considering R users who may used to installing R packages via install.packages(...) (where package names may be a mix of upper- and lower-case letters) from an R session, I believe it may be useful to document that installing R packages via apt-packages requires that the package name be lowercase within r-cran-<package>.

[eitsupi]

Thank you for your contribution!

However, I think it is too topical to mention r-cran-* here (i.e., I believe that this Feature is used more often for purposes other than R package installation).
How about adding to the following section?

devcontainer-features/src/r-apt/README.md

Lines 65 to 82 in 95fb05b

	If you want to install R packages via apt during the container build phase,
	you can use [the `ghcr.io/rocker-org/devcontainer-features/apt-packages` Feature](https://github.com/rocker-org/devcontainer-features/blob/main/src/apt-packages)
	to do so.
	```json
	"features": {
	"ghcr.io/rocker-org/devcontainer-features/r-apt:latest": {},
	"ghcr.io/rocker-org/devcontainer-features/apt-packages:1": {
	"packages": "r-cran-curl"
	}
	},
	"overrideFeatureInstallOrder": [
	"ghcr.io/rocker-org/devcontainer-features/r-apt"
	]
	```
	`ghcr.io/rocker-org/devcontainer-features/apt-packages` is not guaranteed to install after this Feature,
	so be sure to set up [the `overrideFeatureInstallOrder` property](https://containers.dev/implementors/features/#overrideFeatureInstallOrder).

And, it seems that uppercase letters cannot be used in apt package names in the first place.
https://askubuntu.com/questions/1262528/what-are-usable-characters-are-for-package-names

[mthomas-ketchbrook]

Thank you so much for your kind response!

Embarassingly, I had overlooked the r-packages feature when I originally created this PR. The r-packages feature seems to be the most "appropriate" place to install R packages.

I'd be happy to add to the section you referenced, and perhaps add an additional comment to simply point out that the r-packages feature also exists for the purposes of installing R packages.

Would you prefer that I make these changes to the NOTES.md file or the README.md?

[eitsupi]

Thanks for checking this out.

Yes, you are correct, the README is automatically generated from the NOTES, so we need to edit the NOTES.

[mthomas-ketchbrook]

I moved the anecdote & example from the apt-packages Feature NOTES to the r-apt Feature NOTES in a5d2629.

Please let me know if this is what you had in mind, or if there are any additional changes I should make. Thanks!

[eitsupi]

LGTM, thanks!

[eitsupi]

The README has been updated by b24eea2.