DAVdroid (Android) installation instructions

Added client installation instructions for Android as I did earlier for iOS.

Not that it's especially difficult but you need to know Android does not provide support for CardDAV or CalDAV out of the box and look for the best sync app to bridge the gap. Hope this will help save some time for the next Android n00b. ;)

Thank you @netgusto and @rfc2822 for your awesome open source work, much appreciated.