Add memetastic.space

It's uh, it's an instance now.

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

@@ -13,5 +13,6 @@ List of Known Mastodon instances
 | [gay.crime.team](https://gay.crime.team) |N/A|Yes(?)|
 | [gnusocial.me](https://gnusocial.me) |Yes, it's a mastodon instance now|Yes|
 | [icosahedron.website](https://icosahedron.website/) |Icosahedron-themed (well, visually), open registration.|Yes|
+| [memetastic.space](https://memetastic.space) |Memes|Yes|
 
 Let me know if you start running one so I can add it to the list! (Alternatively, add it yourself as a pull request).