Add major version upgrade documentation

[n2ygk]

Is your feature request related to a problem? Please describe.

Not everyone reads the CHANGELOG but they may read the RTFD.IO documentation.

Describe the solution you'd like

Add 2.0 breaking change upgrade instructions/warnings to the documentation.

Describe alternatives you've considered

Make the CHANGELOG more obvious.

Additional context

[Beenhakker]

Looking back at this, and going through the documentation ones more, it is stated there correctly, despite it being a bit cryptic maybe:

"Fill the form as show in the screenshot below, and before saving take note of Client id and Client secret we will use it in a minute."

I did create a PR which hopefully clarifies this change a bit better: https://github.com/jazzband/django-oauth-toolkit/pull/1170