added executor link
## References
-* [MESOS HTTP API design document](https://docs.google.com/document/d/1pnIY_HckimKNvpqhKRhbc9eSItWNFT-priXh_urR-T0/edit)
+* [MESOS/scheduler HTTP API Design](https://docs.google.com/document/d/1pnIY_HckimKNvpqhKRhbc9eSItWNFT-priXh_urR-T0/edit)
+* [MESOS/executor HTTP API Design](https://docs.google.com/document/d/1dFmTrSZXCo5zj8H8SkJ4HT-V0z2YYnEZVV8Fd_-AupM/edit#heading=h.r7o3o3roqg12)
