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.
True enough. I guess the first half of the sentence appeals to folks who are trying to find themes and the second half appeals to those who aren't :)