Aqua considers itself a stale dependency

[baggepinnen]

The readme suggests adding Aqua to the [deps] section of Project.toml, but doing so causes Aqua to complain that Aqua is not loaded and is considered a stale dependency:

Test Failed at /home/fredrikb/.julia/packages/Aqua/HWLbM/src/stale_deps.jl:24
  Expression: result ⊜ true
   Evaluated: ⟪result: 😭 FAILED: ControlSystems 
    Some package(s) in `deps` of ControlSystems are not loaded during via `using ControlSystems`.
    * Aqua [4c88cf16-eb10-579e-8560-4a9242c79595]

I don't want to load Aqua in the package, it should be a test only dependency, and I don't want to turn off detection of stale dependencies. A reasonable approach would perhaps be for Aqua to be self aware and filter itself out as a stale dependency?

[DilumAluthge]

Aqua should not be in deps. I think we just need to fix the README.

Aqua can be in extras and the test target, which allows you to set compat bounds on Aqua.

[hyrodium]

The readme suggests adding Aqua to the [deps] section of Project.toml,

Aqua should not be in deps. I think we just need to fix the README.

Note that the README suggests adding Aqua to [deps] in /test/Project.toml but not /Project.toml. I think it would be nice to have another suggestion to add Aqua to [extras] in /Project.toml.

[fingolfin]

There is no bug here, good.

The suggestion for enhancing the documentation is now in issue #123