Documentation/nb/intel: Add Haswell documentation

At the moment, this just gives some details on the MRC.

Change-Id: I84e8ca2543b2e19b84a24f7d7032a4aedb6e9272
Signed-off-by: Tristan Corrick <tristan@corrick.kiwi>
Reviewed-on: https://review.coreboot.org/c/30356
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

Documentation/northbridge/intel/haswell/index.md

@@ -0,0 +1,8 @@
+# Intel Haswell documentation
+
+This section describes the Intel Haswell architecture as it relates to
+coreboot.
+
+## Proprietary blobs
+
+- [mrc.bin](mrc.bin.md)