Add install instructions for cloning WebDAV component (which I think is best if we make optional). There may be webserver dependencies so let's just call this a plugin of sorts, but different than normal plugins because if it's installed it will be tightly integrated with everything we do. Eventually we may provide a static repository or select a specific sabre-dav version so that we're immune from changes upstream which break stuff we're doing; but for now we'll use the fruux main repository and install using the trunk.
Merge https://github.com/friendica/red into zpull