Update README.md - adding winget

[macel94]

Referencing this comment https://github.com/FiloSottile/mkcert/issues/400#issuecomment-913498460 in the #400 issue, seems like it's ok for the creator of this pkg to also let people use winget.

thanks to @ItzLevvie this can be used. the official commit for winget, that references the packages from this repo release(mkcert), is here if anyone needs to take a look: https://github.com/microsoft/winget-pkgs/commit/0a8b8f08a46f22599c9db65b11b64f211d457b75

I believe this should be included in the docs because it's just really convenient.

[macel94]

@FiloSottile