Update installation and getting started instructions.

[jtigger]

This complements #31, directing the user to the tutorial.

It also updates the instructions for installing the runtime; we've gotten far fewer setup questions when using package managers and these instructions are well-tested.

[sdavids13]

Do you think it would be beneficial to show how to run the tests via IntelliJ in the tests doc?

[jtigger]

I know what I'm about to say sounds like I'm out in left field and totally lost my marbles...

... but I went around the world on this one, last year — walking through the onboarding flow from the Exercism homepage up into hello-world; wrote the entire onboarding in terms of Intellij — and realized that (stay with me, here) the best first experience for newcomers is the command line.

The key insight came when I tried to imagine what it would be like for someone brand new to the language and IDE. Sure, it's absolutely intuitive for me now, but when I rolled back my years of experience on the tool... back to when I first laid eyes on an IDE, I vaguely remember feeling particularly overwhelmed. There's actually quite a learning curce to the unitiated.

The programs our practitioners are writing in Exercism are tiny. They actually won't see a whole lot of benefit from the power tool that is Intellij, at first.

Now, if they want to do any serious programming with Kotlin, they would really want to jump into the IDE and climb that learning curve, no question.

Until then, the command-line tools (and the Gradle support is just great) are a far less steep and therefore more accessible.

See also: exercism/java#132

[sdavids13]

Reasonable enough explanation for me.