Update notification for release 0.43

Based on @pthiess's original draft plus the [release notes](https://github.com/adobe/brackets/wiki/Release-Notes:-0.43) draft.

CC @ryanstewart @dangoor @njx in case you have feedback.

Also -- added README instructions for testing JSON changes like these in-situ in Brackets.
What about `Extension Manager: (Own) Themes Tab`?