README: Mention the project language level setting

Author: damithc

README.md

@@ -11,8 +11,9 @@ Prerequisites: JDK 11, update Intellij to the most recent version.
    1. Click `Open`.
    1. Select the project directory, and click `OK`.
    1. If there are any further prompts, accept the defaults.
-1. Configure the project to use **JDK 11** (not other versions) as explained in [here](https://www.jetbrains.com/help/idea/sdk.html#set-up-jdk).
-1. After that, locate the `src/main/java/Duke.java` file, right-click it, and choose `Run Duke.main()`. If the setup is correct, you should see something like the below:
+1. Configure the project to use **JDK 11** (not other versions) as explained in [here](https://www.jetbrains.com/help/idea/sdk.html#set-up-jdk).<br>
+   In the same dialog, set the **Project language level** field to the `SDK default` option.
+3. After that, locate the `src/main/java/Duke.java` file, right-click it, and choose `Run Duke.main()` (if the code editor is showing compile errors, try restarting the IDE). If the setup is correct, you should see something like the below as the output:
    ```
    Hello from
     ____        _