Improve docs readme file

[alexandra-simeonova]

Docs readme file: https://github.com/SAP/luigi/tree/master/docs

Changes proposed:

• Add a link to the Luigi application example
• Add a link to the development guidelines
• Elaborate on the difference between Luigi Core and Luigi Client with 1-2 sentences.

Reason:
I think a lot of people go straight to "docs" when looking for information. It is also linked at the top of the Luigi landing page. It might be useful to include a little more info there.