Add saythanks.io badge in README

[drbenvincent]

Could be useful to gather user testimonials. See https://saythanks.io

Also add to pyproject.toml, see example here https://github.com/pypa/sampleproject/blob/aeeb50a948addcb712ad4261df472263514991e1/pyproject.toml#L131-L136

[Padarn]

I think this is registered under a single user account?

[drbenvincent]

I was a bit confused by what you meant. But yes, it looks like the badge would direct messages to a single person's GitHub account for example. I'm experimenting with this now, but looks like I'd be the one to set it up and I can add some testimonials to the site if we get any :)