Add note for sbt plugins

[wsargent]

It looks like if a maven-style repository is used with Artifactory, then publishing will fail with a 409 conflict because Artifactory has dump POM consistency checks.

• Artifactory fails to transform pom file scalatra/sbt-scalatra#54
• The target deployment path 'net/virtual-void/sbt-dependency-graph_2.10_0.13/0.7.5/sbt-dependency-graph-0.7.5.pom' does not match the POM's expected path prefix 'net/virtual-void/sbt-dependency-graph/0.7.5'. sbt/sbt-dependency-graph#80

The work around seems to be to use an ivy package for SBT plugins, or disable POM consistency checks.