docs: binderfs: add section about feature files

docs: binderfs: add section about feature files

Document how binder feature files can be used to determine whether a
feature is supported by the binder driver. "oneway_spam_detection" is
used as an example as it is the first available feature file.

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20210715031805.1725878-2-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>