Add libraries to dotnet-projects.md

- [HudlFfmpeg](https://github.com/hudl/HudlFfmpeg) - A/V transcoding framework that helps build complex FFmpeg commands.
- [Mjolnir](https://github.com/hudl/Mjolnir) - [Hystrix](https://github.com/Netflix/Hystrix)-inspired fault tolerance with circuit breakers and thread pools.