Add FAQ to GitHub repo

Author: itscynxx

FAQ.md

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+# Vanilla+ common issues
+
+This will go over a majority of the common issues one might have with Vanilla+, and how to solve them. 
+If you see this, please read it before asking for help. 
+Chances are, your issue is in here. 
+However, if you can't figure it out, feel free to ask for help.
+
+## COMPILE ERROR Undefined variable "AddModTitle"
+
+This error comes up if Vanilla+ is not properly installed or enabled. 
+First, make sure you've followed the install guide correctly. 99% of the cause of this issue is a install that isn't properly set up. 
+If you're sure that it's set up right, check the `enabledmods.json` file in the `R2Titanfall` folder. 
+If you see that `VanillaPlus` is set to `false`, set it to `true`. 
+If you don't see `VanillaPlus`, it isn't installed right.
+
+## Call was unable to find function with name 'NorthstarCodeCallback_GenerateUIPresence'. Is it global?
+
+This error comes up if you have 2.3 or higher of Vanilla+, and 1.19 or lower of Northstar (not the core mods of Northstar, but the files such as `NorthstarLauncher.exe` and `Northstar.dll` found in the `Titanfall2` folder). 
+This is because 2.3 was a patch for a compile error introduced in Northstar 1.20. 
+
+Update Northstar to solve the issue
+
+## COMPILE ERROR Undefined variable "NSPushUIPresence"
+
+This error comes up if you have Northstar 1.20 or higher, and have Vanilla+ 2.2 or lower. 
+The reasoning is similar to the answer above, except instead of an up to date Vanilla+ and out of date Northstar, this is due to an up to date Vanilla+ and an out of date Northstar.
+
+Update Vanilla+ to the latest version to solve the issue
+
+## Random authentication failed error
+
+If you have this error message in game and can skip it by clicking `OK`, this is due to the fact that older versions (pre 2.0) of Vanilla+ used `Northstar.Client`, meaning it would still try (and fail) to authenticate you with Northstar servers. 
+You can simply keep pressing `OK` to ignore it, or update Vanilla+ to the newest version. 
+If updating from a version with this error, remove the `Northstar.Client` mod as it's no longer used