Improve installation instructions for Mac

[kytrinyx]

We have a working, detailed set of instructions for Mac users, for how to install the Exercism command-line client (CLI).

You can see it by going to https://v2.exercism.io:

• Log in
• Select an available track (it doesn't matter which one)
• Click "Join the $LANGUAGE track" (where language is the one you picked)
• Click "continue" in the popup
• Click on the first exercise
• Look for the "Getting Started" box on the right hand side -- this is the walkthrough

Improvements

The current mac walkthrough is excellent if you are a complete beginner feeling intimidated by the whole process. It's pure hell if you're an experienced developer who knows their way around the command-line. We have an open PR that splits the Mac path into "expert" and "beginner": #11

I hope to get this merged shortly.

We need help tweaking the flow and wording of the Mac walkthrough:

• are there any steps that should be broken into multiple steps?
• is there any awkward wording?
• are there places where we are using too many words to say something?

One thing I've been considering is to improve the detailed path through in the following way:

1. The basic detailed path has each step
2. All the reassurance and screenshots get added to a separate optional step that people can get to by clicking

Then when they move on from the reassuring screen they get to the next detailed step without the additional reassurance.

That way people can get the extra help when they need it, without being bogged down if they don't.

[kytrinyx]

I'm going to call this "done". We will likely get improvements and polish as people start using it.