add indie.json for Indie App Store listing

indie.json aims for an open standard for installing indie web apps on platforms. It’s based off the W3C spec for a web app manifest: https://w3c.github.io/manifest/

This file has all the info which allows ownCloud to be listed in app stores which use that standard. Several platforms have voiced interest in working towards a shared app data format & perhaps installer store experience. For example Indie Box, ArkOS, IndiePhone, Cozy and more.

cc @karlitschek @brennannovak @DeepDiver1975 


More info about the work on this common format is at https://indiewebcamp.com/store and soon a first »Indie App Store« will be running.