greybus: update README with info on how to build and contact me.

drivers/staging/greybus/README

 Greybus kernel code
+
+To build against the running kernel (odds are you don't want this):
+	make
+
+To build against a specific kernel source tree (odds are you want this):
+	KERNELDIR=/home/some/random/place make
+
+Any questions / concerns about this code base, please email:
+	Greg Kroah-Hartman <greg@kroah.com>