Expanded 2.1.4 Document the Data

[elisehellwig]

Greatly expands the Document the Data section, including linking out to a bunch of great existing resources on metadata. I decided not to go into explicit examples in the text since metadata structures can vary so much between fields/disciplines.

I thought it might be a good idea to put something about documenting technical/scientific instruments you use (see below). but I couldn't find the right place for it, but I've linked to other resources that discuss it.

If you use scientific instruments or machines to collect data, make sure to
document what instruments you used and any settings associated with those
instruments. This could be something as simple as the output units.