docs: Update README.md include submodule init

[Lewiscowles1986]

The instructions missed the submodule init, likely resulting in a bunch of people unable to follow instructions unless they are familiar with building C projects.

Small fix

Original error:

CMake Error at CMakeLists.txt:328 (add_subdirectory):
  The source directory

    /Users/lewiscowles/Projects/study/stable-diffusion.cpp/ggml

  does not contain a CMakeLists.txt file.

I deleted ggml 😊 and then saw

CMake Error at CMakeLists.txt:328 (add_subdirectory):
  add_subdirectory given source "ggml" which is not an existing directory.