Update setup.md (#79)

* Update setup.md

Shizuku would randomly stop running on GrapheneOS (Pixel 7) despite following all directions, fresh installations, etc., until the Graphene-specific security setting "secure app spawning" was disabled. Unclear if this is necessary on all devices, but is a good thing to try for those facing trouble.

* Update setup.md

Define setting location

shizuku/guide/setup.md

@@ -153,6 +153,11 @@ Do not use the scan feature in MIUI's "Security" app, since it will disable "Dev
 
 Don't click the dialog shows after connecting the USB, because it will change USB usage mode.
 
+#### GrapheneOS 
+
+The Security setting "Secure app spawning" may need to be disabled. 
+Note: GrapheneOS is not an officially supported ROM, but as of January 2023 Shizuku will run on it. 
+
 ### Start via root: cannot start on boot
 
 Please allow Shizuku to run in the background.