Documentation/filesystems: add binderfs

This documents the Android binderfs filesystem used to dynamically add and
remove binder devices that are private to each instance.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
[jc: tweaked markup and added to filesystems/index.rst]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>