Create SPI-Birds primer

[StefanVriend]

The standard protocol is an extensive document that outlines the meaning of variables, but for someone new to SPI-Birds and the way we structure data, it might not be the best entry point. Instead, it would be nice to provide a prime that outlines the basic information needed to work with SPI-Birds data.

This may include (but not limited to):

• a description of how to access, search and request data
• an overview (diagram) of how the data tables are linked
• a description of helpful pieces of code (such as utility functions) that may help you calculate other variables not included in the standard protocol (also see Move optional variables in v2.0.0 to separate file #1)