add continuous integration

[rursprung]

i'd recommend adding a GitHub workflow to build the library to ensure that PRs work. also, it should preferably run cargo audit to validate the dependencies, though this can only be done after fixing #9 as otherwise it'll fail.

here's an example of a workflow which i'm using: https://github.com/rust-embedded-community/tb6612fng-rs/blob/master/.github/workflows/CI.yml
you can ignore the job building the example, this isn't needed here (that's just needed there as it compiles for a different target than the library).

also, it makes sense to enable GitHub dependabot in the same step, both for rust as well as the GH actions. see this example: https://github.com/rust-embedded-community/tb6612fng-rs/blob/master/.github/dependabot.yml