Add note about USERNAME being USERID to README

Author: bboe

README.md

@@ -24,3 +24,7 @@ year old. You may also manually specify a `before` date via:
 ```sh
 deslackify --token TOKEN --before YYYY-MM-DD USERNAME
 ```
+
+__Note__: If no results are found, you may need to replace `USERNAME` with
+`USERID`. You can find the `USERID` by going to `Profile & Account`, and then
+clicking the three dots, and at the bottom, "Copy member ID".