Move rust template app to it's own repository

[okdistribute]

• [x] Make sure the example in rs/ up to date, using best practices for the Rust SDK
• [x] Follow the pattern of other templates -- the repo should start with template-...
  • https://github.com/getditto/template-rust
• [x] Link to the repo in this README
• [x] Delete the code in this repository

[nicholastmosher]

Progress on this issue:

• [x] I've updated the Rust sample project with latest best practices and gotten it ready to publish
  • I originally made a PR on this repo but given that we'll be removing it from here, I went ahead and just moved it into it's new repository 👇
• [x] I created https://github.com/getditto/template-rust as the new standalone repository. It's currently a private repo, but after a quick review we can make it public
• [x] Opened https://github.com/getditto/samples/pull/152 to remove the Rust sample from this repo and add a link to the new sample repo in this repo's README, we can merge this after making the new template app public