The releases are available at: https://github.com/Terradue/rLandsat8/releases
Releases can be installed using devtools
Start an R session and run:
library(devtools)
install_url("https://github.com/Terradue/rLandsat8/releases/download/v0.1-SNAPSHOT/rLandsat8_0.1.0.tar.gz")
library(rLandsat8)
Note the example above install the v0.1-SNAPSHOT release, adapt it to the current release