[EASY/DOC] Document the reasoning around requirement levels in BIDS

[sappelhoff]

          > Should we continue with the field being REQUIRED or change it to be OPTIONAL?

Why was it required in the first place? I don't know if we have this written anywhere, but here's what's in my head when making these decisions:

• REQUIRED: Data can't be interpreted without this information (or the ambiguity is unacceptably high)
• RECOMMENDED: Interpretation/utility would be dramatically improved with this information
• OPTIONAL: People/tools might find it useful to have this information

Every required field is a barrier to somebody using the standard, and you're basically stating "Data without this field might as well not be shared".

Originally posted by @effigies in #981 (comment)