Add Python script documentation to docs

[mvbattista]

Checklist

• I'm reporting a feature request
• I've verified that I'm running youtube-dl version 2019.06.27
• I've searched the bugtracker for similar feature requests including closed ones

Description

For use in a python script, parameters are different than those listed in README.md for command line usage. For example, in README.md:

    --cookies FILE                   File to read cookies from and dump cookie
                                     jar in

When trying to call in a standalone Python script importing youtube_dl, the same functionality is passed in the 'cookiefile' argument.

with youtube_dl.YoutubeDL({'cookiefile': '/path/to/cookies.txt'}) as ytdl:

Documentation on the translation between command line arguments and Python object instantiation arguments will help developers as opposed to searching through library source code.

[dstftw]

List of available programmatic options is clearly mentioned in embedding guide.

[mvbattista]

@dstftw So why can't this in-code documentation be moved to a separate embedding Markdown file?