"Similar Projects" section in documentation

[zenflow]

Describe similarities & differences between this library and others:

1. https://github.com/docker/compose
2. https://github.com/Unitech/pm2
3. https://github.com/godaddy/node-cluster-service
4. https://github.com/kimmobrunfeldt/concurrently