warn that IDOM_FEATURE_INDEX_AS_DEFAULT_KEY=1 will be the default

Author: rmorshea

src/idom/config.py

@@ -8,6 +8,7 @@
 
 from pathlib import Path
 from tempfile import TemporaryDirectory
+from warnings import warn
 
 from ._option import Option as _Option
 
@@ -55,3 +56,14 @@
 
 For more information on changes to this feature flag see: https://github.com/idom-team/idom/issues/351
 """
+
+if not IDOM_FEATURE_INDEX_AS_DEFAULT_KEY.current:
+    warn(
+        "In the next release, the feature flag IDOM_FEATURE_INDEX_AS_DEFAULT_KEY will "
+        "be activated by default. To try this out before the next release simply set "
+        "IDOM_FEATURE_INDEX_AS_DEFAULT_KEY=1 as an environment variable. After this "
+        "change, you can revert to the old behavior by setting it to 0 instead. If you "
+        "have questions or issues with this change report them here: "
+        "https://github.com/idom-team/idom/issues/351",
+        DeprecationWarning,
+    )