Added in annotations.xml files for the SLF4J API so that Kotlin users ha...

I'm not sure if this is of any interest, but the following patch makes it so that the slf4j-api artifact will contain annotations.xml files that are then picked up by the Kotlin build. This means that anyone using Kotlin and wanting to use SLF4J will have an easier time because the compiler is given information about which methods are null-safe - specifically about the fact that LoggerFactory.getLogger() is null-safe and so the returned Logger instance can be used without needing to use the "?." operator everywhere.

The annotations.xml files were all generated by the KAnnotator tool, but I then added in a single additional entry for the return type from LoggerFactory.getLogger().