doc(README): Add --recurse-submodules flag to git clone

[BrianUribe6]

The documentation makes no mention of initializing and updating submodules. A user that is following along the tutorial might not look at the repository to see that calcom directory is a submodule and they need to initialize it before running docker compose.

[codablock]

Not 100% related to this PR, but it might be worth looking into why the submodules are even necessary. It's quite unusual to do this in docker image repos. If there is interest, I can look into this and propose a simpler solution that requires much less regular maintenance.