vout: document the priority rules

modules/video_output/Makefile.am

 voutdir = $(pluginsdir)/video_output
 vout_LTLIBRARIES =
 
+EXTRA_DIST += video_output/README
+
 if HAVE_DECKLINK
 libdecklinkoutput_plugin_la_SOURCES = video_output/decklink.cpp
 libdecklinkoutput_plugin_la_CXXFLAGS = $(AM_CFLAGS) $(CPPFLAGS_decklinkoutput)

modules/video_output/README

+Video output display plugins
+=============================
+
+Priority rules
+---------------
+
+The priorities ranges are as follows since February 2011:
+
+ - less than  20: console (text)
+ - less than  50: plain frame buffers
+ - less than 100: no embedded windowing
+ - less than 150: SW scaling
+ - less than 200: SW chroma conversion
+ - rest: good stuff :-)
+
+No more than one plugin with a given non-zero riority.
+
+On Windows, native outputs are now priorized over XCB (as there now is
+a Win32 port of XCB).