Add mstdn.io

docs/Using-Mastodon/List-of-Mastodon-instances.md

@@ -21,6 +21,7 @@ There is also a list at [instances.mastodon.xyz](https://instances.mastodon.xyz)
 | [mastodon.xyz](https://mastodon.xyz) |N/A|Yes|Yes|
 | [social.targaryen.house](https://social.targaryen.house) |Federates everywhere, quick updates.|Yes|Yes|
 | [masto.themimitoof.fr](https://masto.themimitoof.fr) |N/A|Yes|Yes|
+| [mstdn.io](https://mstdn.io) |N/A|Yes|Yes|
 | [social.imirhil.fr](https://social.imirhil.fr) |N/A|No|Yes|
 | [social.wxcafe.net](https://social.wxcafe.net) |Open registrations, federates everywhere, no moderation yet|Yes|Yes|
 | [octodon.social](https://octodon.social) |Open registrations, federates everywhere, cutest instance yet|Yes|Yes|