Prepare for Nextcloud appstore

[MariusBluem]

Since Nextcloud 11 uses its own App Store (https://apps.nextcloud.com) it would be nice, if you could publish your app there too - so that our community can benefit of this nice addition 👊

• Adjust info.xml for the Nextcloud appstore
• Polishing the README.md a bit

You will find out more about how to publish this app in the Nextcloud appstore over here: https://nextcloudappstore.readthedocs.io/en/latest/developer.html

If you need any help, feel free to ask 😉 @westberliner

Signed-off-by: Marius Blüm marius@lineone.io

[westberliner]

Thx, I wanted to apply my app to the nextcloud "store" earlier. But I didn't have the time yet to do the certificate stuff.