readme: Add '--recursive' to git submodule command

[bemug]

Not doing so results in the following error :

CMake Error at Externals/cubeb/CMakeLists.txt:30 (message):
Could not find sanitizers-cmake: run

git submodule update --init --recursive

in base git checkout

As the advice is correct, update readme with the correct command.

[Florin9doi]

You may add a -j $(nproc) as well, where supported

[bemug]

My fat fingers clicked the close button without my brain wanting to. I apologize for the noise.

@Florin9doi -j$(nproc) is already mentionned further on the readme file.

[Florin9doi]

@Florin9doi -j$(nproc) is already mentionned further on the readme file.

Here it is used to compile using multiple jobs, but the submodule update process may also be speed up by running multiple jobs.

[bemug]

I was not aware that git submodule can take this argument, thanks.

The process was quite fast on my machine. Also as this is a readme I'd rather keep it simple. Thoughts ?

[AdmiralCurtiss]

Yeah, I think this is fine as-is, I don't know how much speedup you can even get by parallelizing submodule checkout.

[v1993]

FWIW, I get quite a bit of speedup with -j4 option, but it probably should not be tied to thread count - the bottleneck of resulting processes is network and disk I/O and you probably don't want to open tons of actively used connections to github at once even if your CPU has a lot of (think 64+) threads.