Mention vignette section on faster model saving for large models

[jgabry]

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• Run unit tests
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Summary

In doc for save_object mention the vignette section on faster model saving for large models.

Copyright and Licensing

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Columbia University

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[jgabry]

Going ahead and merging since this is just a minor doc edit