Document required resolvers

[sslavic]

Please document (in readme or wiki page) required resolvers (for snapshot and future releases).

Note that besides Sonatype OSS repositories where this plugin is published, users of the plugin also have to add all "transitive" resolvers, needed for transitive dependencies, like Mulesoft repo, as sbt doesn't seem to support transitive resolvers (see related SO question and answer http://stackoverflow.com/a/23158200/381140 )