Update and improve README

[BillSwirrl]

The instructions in the README of the repository could be improved to make it easier for new users. Also some of the examples no longer work with the example repository - we should go through and check each one and update them where necessary.

Additional things I would find useful:

• an example showing how to list the dimensions of a dataset (there is one, but it doesn't work)
• an example showing how to list the possible values of one specific dimension
• more explanation of the approach to filtering a dataset by fixing the value of one or more dimensions
• documentation on how to access a graphql endpoint programmatically eg GET /graphql?query= or POST to /graphql with details and an example of what the payload must be
• some hints on how to use the Documentation Explorer of the interactive graphql interface to find out relevant things for using cubiql

[zeginis]

@BillSwirrl I can fix this issue.