Add step to initialize and fetch submodules

[potomak]

I was getting this error:

CMake Error at CMakeLists.txt:28 (add_subdirectory):
  add_subdirectory given source "libsane" which is not an existing directory.

because I didn't know that I had to initialize libsane that is a git submodule.

https://www.git-scm.com/book/en/v2/Git-Tools-Submodules

[ksherlock]

Thanks. I overlooked that when I added the submodule.