Document how to enable checkUnusedDependencies

[ash211]

Before this PR

I mistakenly thought that com.palantir.baseline-exact-dependencies being applied (by default) meant that that check would run checkUnusedDependencies. Actually it does not, and repos wanting this behavior need to opt into it.

After this PR

==COMMIT_MSG==
Document how to enable checkUnusedDependencies
==COMMIT_MSG==

This PR documents how repos can do that, so it's an easy copy+paste if desired.

Possible downsides?

None known

[ash211]

Without this line, my internal test repo fails with:

1: Task failed with an exception.
-----------
* Where:
Build file '/Volumes/git/my-project/build.gradle' line: 77

* What went wrong:
A problem occurred evaluating root project 'my-project'.
> Could not get unknown property 'checkUnusedDependencies' for root project 'my-project' of type org.gradle.api.Project.

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.

Maybe there's an alternate pattern that we should recommend instead, of only applying to subprojects that are Java type?

[pkoenig10]

I probably wouldn't be prescriptive in this documentation about putting this in allprojects - that's something that consumers can decide for themselves.

I'd just write something like:

+ tasks.check.dependsOn tasks.checkUnusedDependencies