Suggestion for cargo book Continuous Integration section to include GitHub Actions

**Describe the problem you are trying to solve**
It would be helpful to have an official example of using cargo with GitHub actions for CI/CD.

**Describe the solution you'd like**
A "GitHub Actions" section in https://doc.rust-lang.org/cargo/guide/continuous-integration.html.

**Notes**
Might be related to https://github.com/rust-lang-nursery/cli-wg/issues/161.
