DeveloperGuide: add solution to src being treated as normal folder (s…

…e-edu#105)

Occasionally, new developers may accidentally import the project as
a Gradle project.

This cause IntelliJ to often triggers Gradle synchronization which
allows Gradle to impose the project to use its configuration. Due
to this issue, developers will encounter the problem of marked
directory keep getting reset upon restarting IntelliJ.

Let's add the solution to this problem in the troubleshooting
section to help the developers who have accidentally misconfigured
the importation.

docs/DeveloperGuide.adoc

@@ -54,7 +54,7 @@ the test has passed
 . If the script reports that there is no difference between `actual.txt` and `expected.txt`,
 the test has passed
 
-*Troubleshooting test failures*
+*Troubleshooting*
 
 * Problem: How do I examine the exact differences between `actual.txt` and `expected.txt`? +
 Solution: You can use a diff/merge tool with a GUI e.g. WinMerge (on Windows).
@@ -66,3 +66,8 @@ OSes. Convert the `actual.txt` to the format used by your OS using some https://
 * Problem: Test fails during the very first time. +
 Solution: The output of the very first test run could be slightly different because the program
 creates a new storage file. Tests should pass from the 2nd run onwards.
+
+* Problem: `AddressBook` cannot be run, and `src/` is treated as a normal folder instead of as a `Sources Root`. +
+Solution: This can happen if you've accidentally imported the project as a Gradle project.
+First, disable `"Use auto-import"` in `Settings (Preferences)` > `Build...` > `Build Tools` > `Gradle`.
+Finally, right click on `src/` > `Mark Directory as` > `Sources Root`.