Announcing the new jfx-sandbox repo

[Kevin Rushforth]

We have created a new jfx-sandbox repo:

https://github.com/openjdk/jfx-sandbox

This repository serves the same purpose as the jdk-sandbox: It allows 
multiple OpenJFX Committers to collaborate on larger shared fixes, 
prototypes, or projects. By way of example, it could be used for a macOS 
Metal pipeline, a GTK 4 platform, or Wayland.

All OpenJFX Committers have direct push access to the jfx-sandbox repo 
(just like the JDK Committers do for jdk-sandbox).

General rules and guidelines:

There are only two hard-and-fast rules that must be observed:

1. Don't push any commits to the master branch and don't open a PR 
against master. The master branch is continuously mirrored from the 
jfx:master branch and should be considered read-only in jfx-sandbox.

2. Don't push to someone else's branch without talking to them first.

And a couple general guidelines that will make this a more pleasant 
place to play:

3. Unless this is intended to be a shared, semi-formal project (e.g., 
metal, gtk4, wayland-x11, wayland-native), please prefix the name of 
your branch with either a JBS bug ID or your openjdk username.

4. Don't force push to a branch without getting consensus among the 
collaborators.

Let me know if you have any questions.

-- Kevin