Improve contributing.md

[olegklimov]

"Thanks. I’m trying to follow the contribution guidelines to install for development. I am able to get the web GUI, inference, and scratchpads up and running but when I upload a git repository and click “scan sources” nothing happens. I do not see any error messages in the logging. Any advice?
When I run the app normally from the docker container everything works"

That's probably "expect" and "apt install expect". But no messages indicating that?

[klink]

@FuzzyReason could you please help to improve the contribution guidelines?

[FuzzyReason]

Hey, @klink!
Yeah, I'll prioritize that. That is important indeed.