Add GTSAM citation

[varunagrawal]

Feature

Generate a bibtex citation for use in academic publishing.

Motivation

We use GTSAM for research, and Github repositories get cited all the time these days.

Pitch

The process is really simple:

1. Link this repo with Zenodo (instructions here). This will create DOIs for each new release. Only @dellaert can do this since it requires admin privileges.
2. Create a bibtex citation in the README as per this tutorial.

Alternatives

NA

Additional context

NA

[dellaert]

Feel free to start a draft about GTSAM 4

[varunagrawal]

If you create the DOI, I can add the badge to the README as well as autogenerate the BibTex via a curl command.

[varunagrawal]

An attempt at humor in this repo. Seemed apt for this issue.

[dellaert]

You're gonna have to me walk through what needs to happen next time we meet

[varunagrawal]

@dellaert we can now finish this. We should do it in our next meeting.