Add data context to documentation

[EOstridge]

Is your feature request related to a problem? Please describe.

As a data consumer, I would like there to be better context for the data this package returns.

Describe the solution you'd like

It would be useful if there was a glossary of terms or data dictionary created to provide this additional context in an easy-to-access place.

Example: NOCs - NOC stands for National Operator Code. Each bus operator has its own identifying code, consisting of four letters. To find the NOC for the operators you are interested in, follow these steps. (Also, consider whether BDE can offer a service where user can check what the code for an operator is, if that's helpful).

ATCO (Association of Transport Co-ordinating Officers) codes refer to the unique identifiers used for individual UK bus stops. The first 3 numbers denote the authority responsible for the stop. The fourth character is a 0 (zero). The remaining characters to a maximum of 8 are alpha-numeric determined locally BODS Data Extractor allows you to filter results on the ATCO admin authority (the first 3 numbers). See fine-tuning section below for more detail on how to do this.

[lldwork]

Please see the BODS data catalogue for relevant field definitions - can just include a link potentially

[lldwork]

include or link to this set of docs where possible https://data.bus-data.dft.gov.uk/guidance/requirements/?section=casestudies

[EOstridge]

Created pull request for glossary