Add more concrete Producer and Operation usage examples

[nkinesis]

• Problem: while we have examples in docs/examples/code and docs/examples/queries, they are not related to more concrete explanations. Therefore, while they exist, they are not very useful and are not very often seen by new CITYdata users.
• Solution: add to our README.md (or create a new .md file) with more use-case-oriented explanations. For example: "Imagine you are a researcher who wants to use data X to do Y, Z and W. To do so, you can use our producer A with our operation B, C and D to achieve this and that results".
• Also, add this to our welcome page.