Update README.md to recommend using OpenIddict for new applications

[kevinchalet]

With most of the pain points addressed in previous OpenIddict releases (e.g WS-Federation claims or authentication schemes mapping) and a lot of services now being supported (78 at the time of writing), the OpenIddict client and its web providers are a good alternative to the packages we maintain here.

As such, I'd like to update the README to encourage users to adopt them for new applications (while still making clear the aspnet-contrib providers are still supported, of course).

[kevinchalet]

Maybe just have a smaller tip at the top, that then links to a section further down with the additional details?

Good idea! 👍🏻

[martincostello]

Looks good 👍

[kevinchalet]

Thanks!