Clarify sorting for Collections

See also radiantearth/stac-browser#431

Author: m-mohr

README.md

@@ -100,6 +100,15 @@ See <https://github.com/stac-api-extensions/query> for details.
 The sort extension can be implemented, too. It works as it does for Items.
 See <https://github.com/stac-api-extensions/sort> for details.
 
+Note that Items and Collections have differences in (common) availability of properties and their location within the document. You can find some examples below:
+
+| Use Case              | STAC Collection | STAC Item |
+| --------------------- | --------------- | --------- |
+| Sort by id            | id              | id |
+| Sort by title         | title           | properties.title |
+| Temporal sorting      | extent.temporal | properties.datetime |
+| Sort by creation time | created         | properties.created |
+
 #### Fields
 
 - Conformance class: `https://api.stacspec.org/v1.0.0-rc.1/collection-search#fields`