Update IntegrationWithExistingApps.md for Swift 3

[ganjianwei]

Update integrating with existing Swift App code for Swift 3.

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@facebook-github-bot shipit

[lacker]

@ganjianwei We need a signed CLA to ship this - would you mind signing it?

[ganjianwei]

Just signed the CLA.

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[lacker]

@facebook-github-bot shipit

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