README.md: add structr/meetdown

Closes #8

README.md

@@ -13,6 +13,7 @@
 | [GetTogether](https://gettogether.community/)         | [GitHub](https://github.com/GetTogetherComm/GetTogether)                                                                      | mature         | Python           | BSD 2-Clause                  |
 | [Gospeak](https://www.gospeak.fr/)                    | [GitHub](https://github.com/loicknuchel/gospeak)                                                                              | mature         | Scala            | Apache 2.0                    |
 | [joind.in](https://joind.in/)                         | [GitHub](https://github.com/joindin/joindin-web2)                                                                             | mature         | PHP              | BSD 3-Clause                  |
+| [Meetdown](https://meetdown.org/)                     | [GitHub](https://github.com/structr/meetdown)                                                                                 | unmaintained   | Structr          | Apache 2.0                    |
 | Meetup Alternative by Thinkmill                       | [GitHub](https://github.com/Thinkmill/meetup-alternative)                                                                     | defunct        | JavaScript       | unlicensed                    |
 | [Mobilizon](https://joinmobilizon.org/en/)            | [framagit.org](https://framagit.org/framasoft/mobilizon/)                                                                     | in development | Elixir           | AGPL                          |
 | [on_ruby](https://www.onruby.eu/)                     | [GitHub](https://github.com/phoet/on_ruby)                                                                                    | mature         | Ruby             | (extended) Beer-ware          |