Add documentation explaining how to start and configure DFaaS agents

[ema-pe]

Currently, there is no documentation describing how to configure and start DFaaS agents, how to generate private keys (together with the corresponding IDs), or how to connect the agents once they are running in each cluster. While there are some comments in the values.yaml file of the DFaaS agent Helm chart, they are not sufficient. This documentation could be improved to enable an easier setup and execution of small experiments, which is especially important for users who want to try DFaaS.