Document order of preference for format selection (closes #798)

README.md

@@ -98,7 +98,8 @@ which means you can modify it, redistribute it or use it however you like.
                                verbose)
 
 ## Video Format Options:
-    -f, --format FORMAT        video format code
+    -f, --format FORMAT        video format code, specifiy the order of
+                               preference using slashes: "-f 22/17/18"
     --all-formats              download all available video formats
     --prefer-free-formats      prefer free video formats unless a specific one
                                is requested

youtube_dl/__init__.py

@@ -165,7 +165,8 @@ def parseOpts(overrideArguments=None):
 
 
     video_format.add_option('-f', '--format',
-            action='store', dest='format', metavar='FORMAT', help='video format code')
+            action='store', dest='format', metavar='FORMAT',
+            help='video format code, specifiy the order of preference using slashes: "-f 22/17/18"')
     video_format.add_option('--all-formats',
             action='store_const', dest='format', help='download all available video formats', const='all')
     video_format.add_option('--prefer-free-formats',