SBT support

@moznion has made a spotless plugin for the sbt buildsystem. I bumped into it the other day, and I began a conversation with the author about the option of upstreaming it https://github.com/moznion/sbt-spotless/issues/22

We should definitely let our users know that it exists by linking to it in the root README. I think it would also be good to upstream it into the spotless repo. The advantage of that is that when we add features / fix bugs in various steps, it would be easier for sbt users to know which versions to use. For spotless users that don't use `sbt`, it helps to have more eyes on the various steps - you never know who might fix a bug or bring a feature.

The downside of having more build steps is that our build gets bigger and slower. Right now, we have the ability to exclude the maven plugin, we could always change it to where all the plugins are excluded, and you have to manually set `SPOTLESS_INCLUDE_GRADLE` or `SPOTLESS_INCLUDE_SBT`, so that developers working on the build system they care about don't get bogged down by the larger scope.

https://github.com/diffplug/spotless/blob/74d964d22636e58e897ff298c6df996e3fac2b69/settings.gradle#L47-L49

The main challenge to upstreaming @moznion's work is just

- a PR with a folder `plugin-sbt`
- add the folder to `settings.gradle`
- get it building via gradle, so that our publishing works https://docs.gradle.org/current/userguide/scala_plugin.html

Comienza con el README raíz, settings.gradle alrededor de las líneas 47-49 y el proyecto sbt-spotless enlazado. Revisa la guía de Gradle para el plugin de Scala y el patrón existente de inclusión en el build; se considera terminado cuando el plugin de SBT está incluido como plugin-sbt, se construye mediante Gradle para su publicación y está enlazado desde el README.

Escrito por el modelo de indexación a partir del texto del issue.