add pointers to other open source DP projects

[pdurbin]

Just like how Google lists OpenDP under "related projects" here...

https://github.com/google/differential-privacy#related-projects

... we should list their project and other open source DP projects.

I'm thinking that "Related Projects" fits nicely in the User Guide, like this:

[pdurbin]

I added Google DP and the page above in 8d09184.

[pdurbin]

In 05e9bc8 I linked to a list from the Handbook on Using Administrative Data for Research and Evidence-based Policy. (Salil let me know about it.) Before adding each project, it would be best to verify that it's still active and happy with a short paragraph describing it.

Another project to consider adding is https://github.com/yuxiangw/autodp