Provide information about test-runner to the user

[Bobbi Towers]

There is a steady trickle of confusion amongst users who bang up against the minor differences between real Clojure and the SCI interpreter that powers the test-runner.

One suggestion from the Clojure community was that we indicate this somewhere. I agreed but that still left the question of where?

I was doing the Javascript Track and noticed some nice comments atop the Lasagna exercise, giving the user some initial notes. I particularly like the // 👋🏽 Hi there!.

We should do something similar, just on the first Concept Exercise should be acceptable. It should also indicate the new spec integration (exercism/clojure-test-runner#35) once that PR is reviewed and merged.

For now it should say something like:

;; 👋🏽 Hi there! Welcome to the Clojure Track.
;; The online test-runner is powered by babashka and the Small Clojure Interpreter (SCI).
;; Almost all language features are supported, with the exception of low-level constructs
;; like `deftype`, and certain Java classes. For more info, see:
;; https://github.com/babashka/babashka#differences-with-clojure