RPM example in README missing `release`

[pamolloy]

Error: release is required, but not set (got: none)!

Also would be nice to link to documentation explaining release vs. version.

See https://github.com/analogdevicesinc/adsp-ldr/actions/runs/17941150018/job/51017912293#step:12:155. Note that the Debian upload was successful with the default keys/attributes copied from the README.