Amend README usage example with string[] cmd

[CyberFoxHax]

Here's a copy paste ready description.

Command formatting

A simple command line looks like this

new String[]{"-i", "input.avi", "-b:v", "64k", "-bufsize", "64k", "output.avi"};

You must split your arguments each into one array element or you'll get errors such as "Unrecognized option" it's not necessary to write "ffmpeg" in the beginning, FFmpeg-Android will add that automatically.

note: the String[] is passed directly onto ProcessBuilder.

• https://docs.oracle.com/javase/7/docs/api/java/lang/ProcessBuilder.html

finally please refer to the official ffmpeg documentation on how to actually use FFmpeg: https://ffmpeg.org/ffmpeg.html

Cheers!