[codex] document submission inspection and deletion flow (#55)

* document submission inspection and deletion flow

* reframe README note as reward hack section

Author: msaroufim

README.md

@@ -124,6 +124,23 @@ popcorn submissions delete <ID>
 popcorn submissions delete <ID> --force
 ```
 
+#### Avoid Reward Hacks
+
+If you want to avoid reward hacks, inspect your past submissions and delete any bad ones.
+
+```bash
+# 1. List your submissions and note the ID
+popcorn submissions list --leaderboard grayscale_v2
+
+# 2. Inspect the exact code stored for that submission
+popcorn submissions show 1234
+
+# 3. Delete the submission if you do not want it kept
+popcorn submissions delete 1234
+```
+
+`list` shows the submission `ID`, `show` prints the full submitted code for that `ID`, and `delete` previews the submission before removing it.
+
 ### Authentication
 
 Register or re-register your CLI with Discord or GitHub.