nl80211: add notes about userspace API/ABI modifications

Add notes about userspace ABI/API modifications, including the
fact that we decided that API submissions should come with a
driver implementation.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/uapi/linux/nl80211.h

@@ -25,6 +25,19 @@
  *
  */
 
+/*
+ * This header file defines the userspace API to the wireless stack. Please
+ * be careful not to break things - i.e. don't move anything around or so
+ * unless you can demonstrate that it breaks neither API nor ABI.
+ *
+ * Additions to the API should be accompanied by actual implementations in
+ * an upstream driver, so that example implementations exist in case there
+ * are ever concerns about the precise semantics of the API or changes are
+ * needed, and to ensure that code for dead (no longer implemented) API
+ * can actually be identified and removed.
+ * Nonetheless, semantics should also be documented carefully in this file.
+ */
+
 #include <linux/types.h>
 
 #define NL80211_GENL_NAME "nl80211"