This a very straightforward process and requires zero code.

1. Create a .yml file in formats/src with the name of your format, name of your file will be its key
2. Add the required information below in your .yml file

# User Facing Name of your Format
name: 'AVI'

# File Extension
extension: '.avi'

# Blob Type, if unsure just make it video/extension
type: 'video/avi'

# If the browser can display this format in a video element
display: false

# Optional Field,
# If your format requires some codec and
# cannot default please add a codec here
defaultCodec: H264

# List of supported codecs
codecs:
  - H264
  - MPEG4

You might need to add additional codecs for your format, to learn more about adding codecs see Codecs

3. Submit Pull Request! Please include the link to the formats ffmpeg docs