Add renovate.json

[jen801]

Guidelines

1. You must run the spotlessApply task before commiting, either through Android Studio or with ./gradlew spotlessApply.
2. A PR should be focused and contained. If you are changing multiple unrelated things, they should be in separate PRs.
3. A PR should fix a bug or solve a problem - something that only you would use is not necessarily something that should be published.
4. Give your PR a detailed title and description - look over your code one last time before actually creating the PR. Give it a self-review.

If you do not follow the guidelines, your PR will be rejected.