libraries/qt5: Update README.

Signed-off-by: Willy Sudiarto Raharjo <willysr@slackbuilds.org>

libraries/qt5/README

@@ -21,6 +21,9 @@ To build the documentation:
 
   DOCS=yes ./qt5.SlackBuild
 
+(you will need the llvm package from extra/ to build
+documentation)
+
 To build the examples:
 
   EXAMPLES=yes ./qt5.SlackBuild
@@ -100,3 +103,32 @@ clarity.
   qca-qt5
   psi-plus
   sddm
+
+============================================================
+
+TROUBLESHOOTING
+
+If Qt5 fails to build, it may be worth using the following
+steps.
+
+1 Uninstall the old Qt5 package
+2 Log out of root
+3 Log back in as root
+4 Build and install the new package
+5 Log out of root
+6 Log back in as root
+7 Upgrade all other packages that depend on Qt5
+
+The reasoning behind all the log outs/ins, is because Qt5
+installs /etc/profile.d/qt5.sh that sets QT5DIR and adds
+QT5DIR/bin to your $PATH when you log in (there's a similar
+file for csh.) Even though the old Qt5 is removepkg'd, those
+settings are *still there* until you log out.
+
+You can see with a test (assuming bash or a zsh type shell):
+
+env | grep -i qt5
+
+If that command is used before step 4, you shouldn't see
+anything qt5 related in PATH and QT5DIR shouldn't exist.
+