Document how to report progress in the python code

As a user it is nice to get some progress when the oSparc computational services are running.
In this particular case one need to log to stdout/stderr entries such as
```bash
progress 0.1
... # some code
progress 0.3
... # some code
progress 0.6
```
This will be picked up by the oSparc backend


This should be documented for the users.