Create README.md for compat layers

core/compat/README.md

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+The compatibility layer is a new core CFW module introduced in ARK-4 that allows the CFW to be booted under different devices and environments.
+
+It serves as a device driver and is loaded right after SystemControl, allowing it to fix or extend the CFW for the device or configuration it's currently running on.
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+The compat layer works completely transparent and can interact with the rest of the CFW, but the rest of the CFW is unaware of the compat layer's inner workings.
+
+While the idea of a compatibility layer is new to the ARK CFW, other well known homebrew already made use of the idea,
+special mentions to `InfinityControl`, `TMControl` and `PopsCore` from `Infinity 2`, `Time Machine` and `PopsLoader` respectively.
+
+The main compat layers for ARK are the following:
+
+- `PSP`: allows ARK to work on retail `PSP` hardware as well as devkits and testkits. Based on `PRO` with some enhancements from `ME`.
+- `Vita`: allows ARK to work on the retail `PS Vita`'s PSP Emulator (aka. `ePSP`). Based on earlier `ARK` versions.
+- `VitaPops`: allows ARK to work on retail `PS Vita`'s PS1 Emulator (aka. `ePSX`), essentially a heavily limited and restricted pspemu. Based on `TN-X` with enhancements from `ARK-3`.
+- `Pentazemin`: allows ARK to work on the modified `PS Vita`'s PSP Emulator running `Adrenaline` (aka. `vPSP`). Based on `Adrenaline`.
+
+Compatibility layers are also responsible for defining the Rebootex binary to be loaded by the CFW on reboot.
+Unlike classic CFW, ARK's SystemControl does not have a rebootex binary built-into it.
+Rebootex is heavily tied to the device and environment running, thus it's also part of the Compat Layer.