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.
We can't use HTML formatting in any of this -- the UI requires it to be plain text.

I disagree with the statement that putting the period inside renders the quote incorrect, though.  In English-style punctuation, it's understood that the period is not part of the quote -- just like it's understood that the quotation marks themselves are not part of it.  There's nothing special about quoting strings from a UI (compared to any other passage of text) that makes it more "incorrect" to format the standard way, IMHO.

But anyway, as I said -- I'll make the change nonetheless.