Point to adoptopenjdk for getting JDK if needed

[Justin]

People often have trouble with instructions like "download the JDK" because, let's face it, the JDK is a confusing thing.

We should link users to an appropriate AdoptOpenJDK download to make it really easy. Of course any other JDK distribution can still be used if desired.

• Tags added: 4.1.0

[Justin]

This has been superseded by #1945: Bundle a stripped down JDK for HTML5 and Android development.

• Status changed from New to Closed
• Tags removed: 4.1.0