Updates readme.md

[purva-thakre]

Updates readme with information about formatting and style. I couldn't find any information about this in qutip's documentation or was looking at the wrong pages.

This instructs a user to install Black and ways to format their code. The defaults set by Black have not been changed but could be edited.If another package is preferred, then I can change the lines for preferred package.

I do think there needs to be additional information regarding the use of Black in documentation and/or test files. Will add this after I get a feedback on this initial PR.

[purva-thakre]

Actually, I think it's better to close this for now.

There's a lot of files that need to be re-formatted. It's better to create a separate issue to handle the errors one by one + verify these changes are needed. Then maybe this information could be added after black doesn't give any errors in existing files.

[BoxiLi]

Yeah, there are many files that are not in the ideal pip8 format at the moment. We can add it as a suggestion for contributors, not a requirement.

If we slowly require all our new code to be pip8 formated. Things will get better.

And if we were to add this, it is better to add it to the contributing to code in the doc, I think, rather than in readme.