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 or JavaFx APIs depending on the
Intellij Edition being used, 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" section to address this issue, and how
to resolve it.

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

docs/DeveloperGuide.adoc

@@ -11,6 +11,7 @@ ifdef::env-github[]
 :tip-caption: :bulb:
 :note-caption: :information_source:
 :warning-caption: :warning:
+:experimental:
 endif::[]
 :repoURL: https://github.com/se-edu/addressbook-level4/tree/master
 
@@ -46,6 +47,10 @@ Do not disable them. If you have disabled them, go to `File` > `Settings` > `Plu
 . Click `OK` to accept the default settings
 . Open a console and run the command `gradlew processResources` (Mac/Linux: `./gradlew processResources`). It should finish with the `BUILD SUCCESSFUL` message. +
 This will generate all resources required by the application and tests.
+. Open link:{repoURL}/src/main/java/seedu/address/storage/XmlAdaptedPerson.java[`XmlAdaptedPerson.java`] and link:{repoURL}/src/main/java/seedu/address/ui/MainWindow.java[`MainWindow.java`] 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=...' to module compiler options` for each error
+. Repeat this for the test folder as well (e.g. check link:{repoURL}/src/test/java/seedu/address/commons/util/XmlUtilTest.java[`XmlUtilTest.java`] and link:{repoURL}/src/test/java/seedu/address/ui/HelpWindowTest.java[`HelpWindowTest.java`] for code errors, and if so, resolve it the same way)
 
 === Verifying the setup