add caution note to README

README.rst

@@ -189,6 +189,13 @@ The `open` function also works as a context handler:
     with asdf.open("example.asdf") as af:
         ...
 
+.. warning::
+    The ``copy_arrays`` argument of `asdf.open()` and `AsdfFile` is deprecated,
+    and will be removed in ASDF 4.0. It is replaced by the more-explicitly-named
+    ``memmap`` argument as the inverse of ``copy_arrays``
+    (``memmap == not copy_arrays``). Using ``copy_arrays`` will work for now, but
+    give an ``AsdfWarning``.
+
 To get a quick overview of the data stored in the file, use the top-level
 `AsdfFile.info()` method: