Add the joind.in link to the README for WavePHP

Author: stevegrunwell

README.md

@@ -29,7 +29,7 @@ This presentation also has [a companion repository, full of executable examples
 
 ## Presentation History
 
-* [WavePHP 2018](https://wavephp.com/) — September 20, 2018
+* [WavePHP 2018](https://wavephp.com/) — September 20, 2018 ([join.in](https://joind.in/talk/6908c))
 * [Southeast PHP](https://southeastphp.com/) - August 17, 2018 ([PDF](https://github.com/stevegrunwell/building-for-php-cli/releases/download/southeastphp-2018/slides.pdf), [joind.in](https://joind.in/talk/ed2e4))
 * [PHPDetroit](https://phpdetroit.io/) - July 28, 2018 ([PDF](https://github.com/stevegrunwell/building-for-php-cli/releases/download/phpdetroit-2018/slides.pdf), [joind.in](https://joind.in/talk/e6d00))
 * [php[tek] 2018](https://tek18.phparch.com/speakers/steve-grunwell/) - May 31, 2018 ([PDF](https://github.com/stevegrunwell/building-for-php-cli/releases/download/phptek-2018/slides.pdf), [joind.in](https://joind.in/talk/c6025))