[lmi] Release notes for lmi-20180719T1706Z

[Greg Chicares]

A new checkbox has been added to the "Preferences" dialog:
  [ ] Overwrite old cells when pasting a census
If you check it, then pasting from a spreadsheet replaces the
former contents. That's often what you want, especially if you're
copying a census out of lmi, editing it in a spreadsheet, and
pasting it back in. If you prefer the old behavior, just leave it
unchecked.