fix: add text around exclude but not include

Author: gadomski

README.md

@@ -98,6 +98,7 @@ example:
     if `properties` is included and `properties.datetime` is excluded, then
     `datetime` should not be in `properties`, but every other nested field should be.
 6. If the same field is present in both `include` and `exclude`, it should be included.
+7. If the a field is not present in `include`, but it is in `exclude`, it should be excluded.
 
 ### `null` vs. empty vs. missing
 
@@ -115,6 +116,7 @@ default set (i.e. the required fields for a valid STAC Item) should be returned.
 | missing | ["a", "b"] | ALL except for ["a", "b"] |
 | `null` or empty | ["a", "b"] | DEFAULT except for ["a", "b"] |
 | ["a", "b"] | ["a"] | ["a", "b"] |
+| ["a"] | ["a", "b"] | ["a"] |
 | ["a.b"] | ["a"] | `a.b` should be the only field in `a` |
 | ["a"] | ["a.b"] | `a` should be included, but it should not include `a.b` |