Add clarification around GET w/o include

[gadomski]

Related Issue(s):

• Closes Ambiguity with GET and no included fields #8

Proposed Changes:

Turns out we already had some text on the subject:

fields/README.md

Lines 185 to 191 in f749a3c

	It is not possible to differentiate between missing, null, and empty for the
	text representation, so implementations should assume the value is empty, NOT
	missing. For example, this is a case of `include` being empty (NOT missing):
	```text
	?fields=-geometry
	```

All we needed was some text in the rule list.

PR Checklist:

• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.