Add doc/idfa.md

Author: wellle

doc/idfa.md

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+## Remove IDFA support
+
+If your app got rejected by Apple because your app is using the advertising
+identifier, this document is for you.
+
+1. Contact us at [[email protected]](mailto:[email protected]). We would like
+   to make sure that you are aware of the consequences of removing IDFA
+   support.
+
+2. After you talked with us, or when you just can't wait any longer, proceed
+   with the following steps to remove the IDFA support.
+
+### Remove the AdSupport framework
+
+- In the Project Navigator select your project. Make sure your target is
+  selected in the top left corner of the right hand window.
+
+- Select the `Build Phases` tab and expand the group `Link Binary with
+  Libraries`.
+
+- Select the `AdSupport.framework` and press the `-` button to remove it.
+
+- In the Project Navigator, expand the group `Frameworks`.
+
+- If the `AdSupport.framework` is in there, right click it, select `Delete` and
+  confirm `Remove Reference`.
+
+### Add the compiler flag `ADJUST_NO_IDFA`
+
+- In the Project Navigator select your project. Make sure your target is
+  selected in the top left corner of the right hand window.
+
+- Select the `Build Settings` tab and search for `Other C Flags` in the search
+  field below.
+
+- Double click on the right side of the `Other C Flags` line to change its
+  value
+
+- Press on the `+` button at the bottom of the overlay and paste the following
+  text into the selected line:
+
+    ```
+    -DADJUST_NO_IDFA
+    ```