[docs][powered by] add airbnb and chronos

docs/Powered-by-Mesos.md

@@ -4,9 +4,11 @@ Organizations using Mesos:
 * [Conviva](http://www.conviva.com)
 * [UCSF](http://www.ucsf.edu)
 * [UC Berkeley](http://www.berkeley.edu)
+* [Airbnb](https://www.airbnb.com)
 
 Software projects built on Mesos:
 
 * [Spark](http://www.spark-project.org) cluster computing framework
+* [Chronos](http://airbnb.github.io/chronos/) distributed and fault-tolerant scheduler
 
-If you're using Mesos, please add yourself to the list above, or email dev@mesos.apache.org and we'll add you!
+If you're using Mesos, please add yourself to the list above, or email dev@mesos.apache.org and we'll add you!