update notification for release 0.42

Any feedback? Thanks!
@peterflynn @pthiess @JeffryBooher @ryanstewart @redmunds @dangoor @RaymondLim @larz0 
+1, I think that's pretty important.  In the Release Notes, I put:

> Built-in dark mode: Choose the built-in "Brackets Dark" theme for easy access to a nighttime color palette.