Fix incorrect usage in the readme examples

[PopGoesTheWza]

• add JSON example to Usage section - Fixes Add POST with JSON data example to documentation #1156
• add description for .paginate.all() - Fixes paginate.all is not mentioned in the docs #1198
• fix sample code under Proxies - Fixes readme.md Proxies section has invalid agent object #1177

Checklist

• I have read the documentation.
• I have included a pull request description of my changes.
• I have included some tests.
• If it's a new feature, I have included documentation updates.

[szmarczak]

Nice work 🙌 I left some hints. Thanks for contributing!

[PopGoesTheWza]

Aye aye, Sir.