Add backtraces feature removal to migrating doc

Author: chipshort

MIGRATING.md

@@ -21,7 +21,9 @@ major releases of `cosmwasm`. Note that you can also view the
 
   If you were using cosmwasm-std's `ibc3` feature, you can remove it, as it is
   the default now. Depending on your usage, you might have to enable the
-  `stargate` feature instead, since it was previously implied by `ibc3`.
+  `stargate` feature instead, since it was previously implied by `ibc3`. Also
+  remove any uses of the `backtraces` feature. You can use a `RUST_BACKTRACE=1`
+  env variable for this now.
 
 - `ContractInfoResponse::new` now takes all fields of the response as
   parameters: