Add link to AppImages to README

[TheAssassin]

Quite a while ago, AppImages have been introduced as distribution format (see #162). Now, the download page shows a link to the AppImage rather than a distro list. To me, it appears that AppImage has evolved to the primary format for shipping the client to end users. However, this isn't reflected in the README of this repository, there is not even a single note mentioning the AppImage.

I'd like to suggest that you at least mention the AppImage, so it can be found from this repository as well.
(I could also imagine that you upload the AppImages to the GitHub releases page, but that's another issue for another day.)

CC @rullzer @probonopd

[rullzer]

Ah yes. I should do that. Do note that we are in the process of switching development to https://github.com/nextcloud/client

[TheAssassin]

Then please add a link to that from this repository, so people know about it.