CB-12000 - Add documentation for --buildFlag cli option

 This closes apache#654

www/docs/en/dev/guide/platforms/ios/index.md

@@ -186,6 +186,13 @@ build configuration file:
 }
 ```
 
+## Xcode Build Flags
+
+If you have a custom situation where you need to pass additional build flags to Xcode -- you would use one or more `--buildFlag` options to pass these flags to `xcodebuild`. If you use an `xcodebuild` built-in flag, it will show a warning.
+
+    cordova build --device --buildFlag="MYSETTING=myvalue" --buildFlag="MY_OTHER_SETTING=othervalue"
+    cordova run --device --buildFlag="DEVELOPMENT_TEAM=FG35JLLMXX4A" --buildFlag="-scheme TestSchemeFlag"
+
 ## Debugging
 
 For details on the debugging tools that come with Xcode, see this [article](https://developer.apple.com/support/debugging)