Documentation: add doc explaining VGUI2 support status, and how to enable it

Documentation/vgui2-support.md

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+# Experimental VGUI2 Support
+
+Thanks to @kungfulon contributions, Xash3D FWGS now supports VGUI2.
+
+VGUI2 is undocumented UI system from Source Engine that GoldSrc used in some games and it's main menu.
+
+## Support status
+
+Doesn't draw anything, doesn't take any input.
+
+## List of games that require VGUI2 support
+
+- Counter-Strike 1.6
+- Counter-Strike: Condition Zero
+- Counter-Strike: Condition Zero Deleted Scenes
+- Day of Defeat
+- Half-Life: Blue Shift
+
+## How to enable VGUI2 support
+
+By default, engine doesn't initialize VGUI2 to not confuse existing games and mods that use or do not use VGUI1. This support also relies on a number of proprietary libraries, that we better not redistribute, so you have to supply them yourself. Keep in mind, that these libraries are compiled only for 32-bit Windows, Linux and OSX, you shouldn't expect them running anywhere else.
+
+1. Download latest Half-Life from Steam.
+2. Open local files.
+3. Copy libraries in the list below according to your platform, to folder where Xash3D searches it's libraries (usually in same folder where executable is located)
+
+| Linux    | Windows
+| -------- | --------
+| `chromehtml.so` | `chromehtml.dll`
+| `vgui2.so` | `vgui2.dll`
+| `libcef.so` | `libcef.dll`
+| `libtier0.so` | `tier0.dll`
+| `libvstdlib.so` | `vstdlib.dll`
+| `libsteam_api.so` | `steam_api.dll`
+
+Some files must also be copied:
+- `cef_gtk.pak`
+
+Additionally, on Linux other libraries must be provided. Use `ldd` tool to figure out which libraries you miss. Many of them can be pulled from Steam Runtime.
+
+Do not recommend using RoDir with VGUI2 games. Many of them are broken. Steam API will crash without `steam_appid.txt` in Xash root directory.