Mention vignette section on faster model saving for large models

[jgabry]

Submission Checklist

• [x] Run unit tests
• [x] Declare copyright holder and agree to license (see below)

Summary

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

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Columbia University

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• 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