Update the README.md (development status etc)

[crspybits]

It would be nice to have a few notes in the README about the Development Status. e.g., To know if it's worthwhile to try this even as an alpha project. Thanks!

[michaelstingl]

@crspybits This SDK powers the production ownCloud app that's available in the public App Store:

• https://apps.apple.com/app/id1359583808
• https://github.com/owncloud/ios-app

Here you can see what features are already implemented, and what's next on the roadmap:
owncloud/ios-app#94

Here you can get in touch with the development team:
https://talk.owncloud.com/channel/mobile
(login with GitHub.com)

[crspybits]

Cool-- Thanks. From "WORK IN PROGRESS" in the README, I didn't realize.