README.md: Improve submodule instructions

[StephanTLavavej]

Part 1

The README's How To Run The Tests With A Native Tools Command Prompt says:

Invoke git submodule update --init llvm-project at the root of the STL source tree.

Miya Natsuhara (@mnatsuhara) observed that this can seem to hang for an eternity with no output. llvm-project is ~700 MB, but the README doesn't warn about this. We should probably add the --progress option here.

Once the README is updated to use the vcpkg submodule (#761, #873), we should consider adding --progress on the git submodule update --init vcpkg command as well.

✔️ Status: Implemented by #1441.

Part 2

Additionally, How To Build With A Native Tools Command Prompt says:

Invoke git clone https://github.com/microsoft/STL

Casey Carter (@CaseyCarter) noted that we should consider adding the --recurse-submodules option here. We shouldn't need to pass the --progress option to git clone explicitly. The How To Build With The Visual Studio IDE instructions would continue to depict separate cloning and submodule init-updating.