Add README.AddingFirmware file. Basically telling people not to.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

firmware/README.AddingFirmware

+
+	DO NOT ADD FIRMWARE TO THIS DIRECTORY.
+	======================================
+
+This directory is only here to contain firmware images extracted from old
+device drivers which predate the common use of request_firmware().
+
+As we update those drivers to use request_firmware() and keep a clean
+separation between code and firmware, we put the extracted firmware
+here.
+
+This directory is _NOT_ for adding arbitrary new firmware images. The
+place to add those is the separate linux-firmware repository:
+
+    git://git.kernel.org/pub/scm/linux/kernel/git/dwmw2/linux-firmware.git
+
+That repository contains all these firmware images which have been
+extracted from older drivers, as well various new firmware images which
+we were never permitted to include in a GPL'd work, but which we _have_
+been permitted to redistribute under separate cover.
+
+To submit firmware to that repository, please send either a git binary
+diff or preferably a git pull request to:
+      David Woodhouse <dwmw2@infradead.org>
+
+Your commit should include an update to the WHENCE file clearly
+identifying the licence under which the firmware is available, and
+that it is redistributable. If the licence is long and involved, it's
+permitted to include it in a separate file and refer to it from the
+WHENCE file.
+
+Ideally, your commit should contain a Signed-Off-By: from someone
+authoritative on the licensing of the firmware in question (i.e. from
+within the company that owns the code).