Suggest using `exclusiveContent` for Gradle 6.2+

[eigenraven]

Gradle 6.2 introduced exclusiveContent blocks, which are even stricter filters that assert that no other repository contains the given artifact filter in addition to asserting that this repository only contains the filtered contents. Wrapping cursemaven in exclusiveContent stops lookups in other repositories for curse.maven artifacts, which can give a performance boost when resolving dependencies:

exclusiveContent {
    forRepository {
        maven {
            name "CurseMaven"
            url "https://cursemaven.com"
        }
    }
    filter {
        includeGroup "curse.maven"
    }
}

I suggest this option should be documented in the readme and front page.