Add `readonly` indicator also for !&modifiable

While typically both are set (e.g. with `:help`), I am using the
modifiable setting alone to prevent editing of some files via an
autocommand (`*.css`, if `*.scss` exists).

I can also set `readonly` for this buffer, but that gets reset when
reloading the buffer, which is not the case for `modifiable`.
Merge pull request #586 from blueyed/readonly-for-nomodifiable

Add `readonly` indicator also for &nomodifiable