mmalomx: Add a README to say this library is deprecated.

Author: 6by9

interface/mmal/openmaxil/README.md

@@ -0,0 +1,10 @@
+Please note that this wrapper layer has never been used in anger,
+and this is NOT used by the firmware.
+
+It was written with the intent of removing native OpenMaxIL support
+from a platform, but that never happened and is not relevant to the
+Raspberry Pi.
+
+Please consider that these files are deprecated. They are maintained
+here in case others are using them, but it is strongly recommended
+that they are not used for new projects.