DeveloperGuide: update "Setting up" section

Due to an ongoing issue[1] with some of the newer versions of IntelliJ,
the IDE may report code errors for JAXB APIs, although the code can be
build and run successfully since we have already imported the
dependencies through gradle.

This may cause confusion amongst users.

Let's update the "Setting up" to address this issue, and how to
resolve it.

[1]: https://youtrack.jetbrains.com/issue/IDEA-189060

docs/DeveloperGuide.adoc

@@ -4,6 +4,7 @@
 :toc-title:
 :imagesDir: images
 :stylesDir: stylesheets
+:experimental:
 
 == Setting up
 
@@ -25,6 +26,9 @@
 . Click `OK` to accept the default settings
 . Run the `seedu.addressbook.Main` class (right-click the `Main` class and click `Run Main.main()`) and try executing a few commands
 . Run all the tests (right-click the `test` folder, and click `Run 'All Tests'`) and ensure that they pass
+. Open the `StorageFile` file and check for any code errors
+.. Due to an ongoing https://youtrack.jetbrains.com/issue/IDEA-189060[issue] with some of the newer versions of IntelliJ, code errors may be detected even if the project can be built and run successfully
+.. To resolve this, place your cursor over any of the code section highlighted in red. Press kbd:[ALT + ENTER], and select `Add '--add-modules=java.xml.bind' to module compiler options`
 
 == Design