add slack

Author: swyxio

MIGRATING.md

@@ -112,6 +112,7 @@ Old content that is possibly out of date
 - [Lyft](https://eng.lyft.com/typescript-at-lyft-64f0702346ea)
 - [Google](http://neugierig.org/software/blog/2018/09/typescript-at-google.html)
 - [Tiny][tiny] - [Talk from ForwardJS here](https://www.slideshare.net/tiny/porting-100k-lines-of-code-to-typescript)
+- [Slack](https://slack.engineering/typescript-at-slack-a81307fa288d)
 
 ## Links