recipes-rt: add a README documenting the contents of recipes-rt

Document the non-obvious bits of using the recipes-rt recipes.

(From OE-Core rev: 2c86e636972bd02b35523b36bc81a2765f430f70)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

diff --git a/meta/recipes-rt/README b/meta/recipes-rt/README
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+The recipes-rt recipes provide package and image recipes for using and testing
+the PREEMPT_RT kernel. The core-image-rt*.bb images are minimal images with
+a couple extra packages, including rt-tests. In order to build the image with
+the linux-yocto-rt kernel, be sure to include the following line in your
+local.conf, bblayers.conf, or your $MACHINE.conf.
+
+PREFERRED_PROVIDER_virtual/kernel = "linux-yocto-rt"
+
+If you are creating a new BSP which should use linux-yocto-rt by default,
+use the line above in the $MACHINE.conf in your BSP layer, and specify the
+following in a linux-yocto-rt bbappend recipe:
+
+COMPATIBLE_MACHINE_$MACHINE = $MACHINE