docs: commonts on mkvtoolnex dependency

Author: rkdarst

README.md

@@ -47,6 +47,9 @@ It depends on the `ffmpeg` command line utility, which must be
 installed through the operating system.  Version requirements of
 `ffmpeg` are currently unknown.
 
+The mkv file properties (title, description, chapters, subtitles)
+require mkvtoolnix to be installed.  This is used by default, but the
+option `--no-mkv-props` will disable this.
 
 
 ## Usage

ffmpeg_editlist.py

@@ -239,7 +239,7 @@ def main(argv=sys.argv[1:]):
     parser.add_argument('--wait', action='store_true',
                         help='Wait after each encoding (don\'t clean up the temporary directory right away')
     parser.add_argument('--no-mkv-props', action='store_false', default=True, dest='mkv_props',
-                        help="Don't try to encode extra properties into the mkv file.  This requires mkvnixtools to be installed")
+                        help="Don't try to encode extra properties into the mkv file.  This requires mkvtoolnix to be installed")
     parser.add_argument('--list', action='store_true',
                         help="Don't do anything, just list all outputs that would be processed (and nothing else)")