Document external referencing

[schymans]

Document how to add external references in equation and variable definitions, e.g. url, doi or literature references.

[schymans]

The inbuilt equations have references in the style :cite:`bibkey`, which are linked to bibtex-type entries in refs.bib. Could something like this be also done on the fly, as in a jupyter notebook?

[schymans]

By the way, instructions should be included in __init__.py, which will then be automatically rendered into https://environmentalscience.github.io/essm/.