Add Joind.in to the thank you slide

Author: stevegrunwell

slides/thank-you.md

@@ -3,4 +3,6 @@
 Steve Grunwell<br>
 Senior Software Engineer, Liquid Web
 
-[stevegrunwell.com/slides/php-cli](https://stevegrunwell.com/slides/php-cli)<!-- .element: class="slides-link" -->
+<p class="slides-link">
+    [stevegrunwell.com/slides/php-cli](https://stevegrunwell.com/slides/php-cli)<br>[joind.in/talk/b9a05](https://joind.in/talk/b9a05)
+</p>