Mention that older editions continue to get most new features

Author: Manishearth

_posts/2018-03-12-roadmap.md

@@ -176,6 +176,8 @@ introduce new keywords or otherwise require adjustments to code. However:
     (rustfix). **Only a small minority of crates should require _any_
     manual work to opt in to a new edition**, and that manual work must be
     minimal.
+-   Many of the features enabled by the new edition will be enabled for the older
+    one as well; older editions continue to get most new features.
 
 In other words, the progression of new compiler versions is independent from
 editions; you can migrate at your leisure, and don’t have to worry about ecosystem