Add a project badge image and display on the README.
This gives people a way to show they use Flake8
as the linter for their project,
and link to the Flake8 project page.
This may be a fun way for people to show their support,
and help drive new users to Flake8.
The certificate at that URL is for `readthedocs.io`, so if you want to use https, it's best to use `flake8.readthedocs.io` and not `flake8.pycqa.org`. As it is currently, you get a certificate error.
Note also that the `Code of Conduct` and `Getting Started Contributing` links are using `http` and pointing to the insecure domain, so if you do want https, you should change those as well.
As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:
> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.
Test Plan: Manually visited all the links I’ve modified.
