Updated Custom campaigns (markdown)

Custom-campaigns.md

@@ -265,6 +265,27 @@ Correct:
       aircraft:
         - F-14B Tomcat
 ```
+### Defining settings
+Any setting can be defined as a suggested default in the campaign.
+
+The syntax is as follows : 
+
+```
+name: Syria - The Long Road to H3
+theater: Syria
+squadrons:
+...
+# Override settings
+settings:
+  # Enable Supercarrier
+  supercarrier: true
+  # Set mission duration to 45 minutes
+  desired_player_mission_duration: 45
+  #  Set max frontline width to 30 km
+  max_frontline_width: 30
+```
+
+For the name and values a setting can take, refer to the user interface and the [settings file.](https://github.com/dcs-retribution/dcs-retribution/blob/dev/game/settings/settings.py)
 #
 ## Designing campaigns in the mission editor