[BUG] Update BUILDING.md with git submodule instruction

[venezuela01]

Code of conduct

• [X] I have read and agree to adhere to the Code of Conduct

Self-training on how to write a bug report

• [X] I have learned how to write a bug report

Is there an existing issue for this?

• [X] I have searched the existing issues

Current Behavior

Now Session iOS with libsession integration is official released, should we update https://github.com/oxen-io/session-ios/blob/master/BUILDING.md to include the libsession git submodule as well?

Expected Behavior

No response

Steps To Reproduce

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iOS Version

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Session Version

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Anything else?

No response

[mpretty-cyro]

I've added a brief section including the command needed to update the submodules as part of #962, this will be merged into master as part of the next release