More info about the index

Author: tgross35

text/3416-feature-metadata.md

@@ -257,7 +257,21 @@ attempt to present them in the following way:
 For features that require new information in the index, it should be possible to
 add this information under a `features3` key. Older versions of Cargo will
 ignore this key, newer Cargo would be able to merge `features`, `features2`, and
-`features3`.
+`features3`. `features3` should mirror the most complete syntax of the
+`[features]` table, i.e.:
+
+```json5
+"features3": {
+    "bar": {
+        enables: ["foo"],
+        deprecated = { since = "1.2.3", note = "don't use this" }
+    }
+}
+```
+
+In order to conserve index space, default keys should be omitted. `Cargo` should
+ignore unrecognized keys within a feature, to allow for future additions without
+needing a new `features` section.
 
 # Drawbacks