Add Discord badge to README

[falbrechtskirchinger]

Add the Shields.io Discord badge with a permanent invite link to the README.

To use the Discord badge a Discord server admin must enable the widget setting on the server.

[coveralls]

Coverage remained the same at 100.0% when pulling 530bdcb on falbrechtskirchinger:discord into df98ca2 on nlohmann:develop.

[nlohmann]

I think we should make clear that Discord will not be a support channel, but a means to coordinate the development. A badge is fine, but this should also go to the README.

[nlohmann]

FYI: Enabled the widget in the server settings.

[nlohmann]

We should mention in https://github.com/nlohmann/json#support that the Discord channel is to coordinate library development and not meant as a means to provide support.

[falbrechtskirchinger]

If the server starts attracting a healthy number of people, I don't see why we couldn't add a dedicated support channel?

[nlohmann]

If the server starts attracting a healthy number of people, I don't see why we couldn't add a dedicated support channel?

I am hesitant, because I still get a lot of emails from people asking support, and I stopped answering (and only referred them to GitHub), because such a 1:1 communication has no benefit to others.

Let's just see what happens now, and we can still change our policy later.

[nlohmann]

Looks good to me.