The manual may be found here https://alexanderrobitzsch.github.io/srm/
The official version of srm is hosted on CRAN and may be found here. The CRAN version can be installed from within R using:
utils::install.packages("srm")
The version hosted here is the development version of srm. The GitHub version can be installed using devtools as:
devtools::install_github("alexanderrobitzsch/srm")