Document lack of support for classifiers

[mjustin]

Per #67, classifiers are not supported in the dependency management plugin. However, the only thing in the documentation that relates to classifiers is specifically talking about their use in exclusions, and not their use in a dependency version:

Gradle does not provide an API for accessing a dependency’s classifier during resolution. Unfortunately, this means that dependency management-based exclusions will not work when a classifier is involved.

It would be helpful if the documentation made it clear that classifiers cannot be specified on dependencies, and not just that classifiers don't work with exclusions.

For instance, the following does not pick the version of the dependency with the "general" classifier (and fails to do so silently):

dependencyManagement {
    dependencies {
        dependency 'com.querydsl:querydsl-apt:4.2.2:general' // INVALID
    }
}

Related Stack Overflow issue