Documentation: document bug compatibility mode

Documentation/bug-compatibility.md

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+# Bug-compatibility in Xash3D FWGS
+
+Xash3D FWGS has special mode for games that rely on original engine bugs.
+
+In this mode, we emulate the behaviour of selected functions that may help running mods relying on engine bugs, but enabling them by default may break majority of other games.
+
+At this time, we only have implemented GoldSrc bug-compatibility. It can be enabled with `-bugcomp` command line switch.
+
+## GoldSrc bug-compatibility
+
+### Emulated bugs
+
+* `pfnPEntityOfEntIndex` in GoldSrc returns NULL for last player due to incorrect player index comparison
+
+### Games and mods that require this
+
+* Counter-Strike: Condition Zero - Deleted Scenes