README: Initialize a readme file

[Aniketh01]

The readme could contain details about:

• How to compile and run the code.
• what the code does.
• why this is prepared etc.

[Aniketh01]

This is a great resource to build your understanding of the project: https://github.com/leandromoreira/ffmpeg-libav-tutorial

You can leverage that for the readme preparation as well.