Add macOS hints

Why this change?

Installation was hard on macOS, and this helped me and other to fix it.

Description

It seems Java 1.7 (which I had installed) or 11 (the latest version) don't play well together with this repo. The description suggests a fix using Android Studio's own Java.

Steps to Test or Reproduce

Running npm run android when your java version is 1.7 produces errors.

Reviewing

Here's what I am looking for feedback on in particular:

• typos
• Is there an easier way?