This has been requested many times. We are pretty ok with rainloop specially that it offers quite the same amount of features (also gpg) plus it integrates with nextcloud. Though in light of autocrypt we might want to consider deploying it. However we do intend to not get to situation where we need to support multiple webmails like rainloop, roundcube, mailpile, etc to satisfy everyone out there. Specially if the best experience and true privacy you get form running mailpile straight form your desktop.
Atm. We are testing it locally as daily driver to see whether its good enough to use it.