Please read the vignette.
Or, after installation, the help page:
#> Mock the Unix Make Utility
#> 
#> Description:
#> 
#>      Use R as a minimal build system. This might come in handy if you
#>      are developing R packages and can not use a proper build system.
#>      Stay away if you can (use a proper build system).
#> 
#> Details:
#> 
#>      You will find the details in
#>      'vignette("An_Introduction_to_fakemake", package = "fakemake")'.
You can install fakemake from gitlab with: