Update README to mention config auto-complete

[3flex]

Config auto-complete should be available in IntelliJ/Android Studio soon (if not already) thanks to SchemaStore/schemastore#1301

Info can be added to the README stating this should work automatically. There are some future enhancements that can be made, the main one being to auto-generate the schema and host it in detekt's repo instead of at Schema Store so there's more control & it's kept up to date automatically, but in the meantime this should work reasonably well. Until it's auto-generated the schema will need to be updated with PRs to the Schema Store repo - that note can be added to the contributing guidelines as well.