CHANGELOG.rst: Add vim text width override

If you have vim set up to recognize `.editorconfig` files, the 80-char limit
from ours causes vim to keep chopping lines. This makes it ignore that limit
when editing `CHANGELOG.rst`

CHANGELOG.rst

@@ -3350,3 +3350,5 @@ to call ttyname() on OS X.
 
 We’ve got some work to do to be as lean as bash, but we’re on the right
 track.
+
+.. vim: ft=rst : tw=0 :