Added placeholder in `inst` directory
Added placeholder in `inst` directory

This is needed to keep the `inst` directory intact on github since the R install process tries to copy files to this folder. `placeholder.txt` will disappear once we add more to the `inst` directory.

cafreeman/sparkGLM,AlteryxLabs/sparkGLM,AlteryxLabs/sparkGLM,cafreeman/sparkGLM