Clarify what an alternate asset it #13

README.md

@@ -9,8 +9,8 @@
 
 The Alternate Assets extension to STAC provides a way to specify alternate locations (e.g., URLs) for assets. Sometimes, assets can be retrieved
 via multiple methods. For example an asset on AWS may have a public facing http URL but also a direct access s3 URL. Or an asset is mirrored
-on multiple servers that have different URLs. In this case the asset is exactly the same, but different protocols or cloud services may be used
-to access it.
+on multiple servers that have different URLs. The file the asset points to must always be exactly the same (i.e. both assets have the same checksum),
+but different protocols, authentication mechanisms or cloud services may be used to access it.
 
 - Examples:
   - [Item example](examples/item.json): Shows the basic usage of the extension in a STAC Item