Updated README file

BUILD.md

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+# Build Instructions
+
+## Windows x86.
+
+### Prerequisites
+
+Install and run [Visual Studio Installer](https://visualstudio.microsoft.com/downloads/). The installer allows you to choose specific components. Select `Desktop development with C++`. You can untick everything you don't need in Installation details, but you must keep `MSVC` ticked. You may also keep `C++ CMake tools for Windows` ticked as you'll need **cmake**. Alternatively you can install **cmake** from the [cmake.org](https://cmake.org/download/) and during installation tick *Add to the PATH...*.
+
+### Opening command prompt
+
+If **cmake** was installed with Visual Studio Installer, you'll need to run `Developer command prompt for VS` via Windows `Start` menu. If **cmake** was installed with cmake installer, you can run the regular Windows `cmd`.
+
+Inside the prompt navigate to the hlsdk directory, using `cd` command, e.g.
+```
+cd C:\Users\username\projects\Client-Mod
+```
+
+Note: if Client-Mod is unpacked on another disk, nagivate there first:
+```
+D:
+cd projects\Client-Mod
+```
+
+### Building
+
+Сonfigure the project:
+```
+cmake -A Win32 -B build
+```
+Note that you must repeat the configuration step if you modify `CMakeLists.txt` files or want to reconfigure the project with different parameters.
+
+The next step is to compile the libraries:
+```
+cmake --build build --config Release
+```
+`hl.dll` and `client.dll` will appear in the `build/dlls/Release` and `build/cl_dll/Release` directories.
+
+If you have a mod and want to automatically install libraries to the mod directory, set **GAMEDIR** variable to the directory name and **CMAKE_INSTALL_PREFIX** to your Half-Life or Xash3D installation path:
+```
+cmake -A Win32 -B build -DGAMEDIR=mod -DCMAKE_INSTALL_PREFIX="C:\Program Files (x86)\Steam\steamapps\common\Half-Life"
+```
+Then call `cmake` with `--target install` parameter:
+```
+cmake --build build --config Release --target install
+```
+
+#### Choosing Visual Studio version
+
+You can explicitly choose a Visual Studio version on the configuration step by specifying cmake generator:
+```
+cmake -G "Visual Studio 16 2019" -A Win32 -B build
+```
+
+### Editing code in Visual Studio
+
+After the configuration step, `CLIENT-MOD.sln` should appear in the `build` directory. You can open this solution in Visual Studio and continue developing there.
+
+## Linux x86
+
+### Prerequisites
+
+Install C++ compilers, cmake and x86 development libraries for C, C++ and SDL2. On Ubuntu/Debian:
+```
+sudo apt install cmake build-essential gcc-multilib g++-multilib libsdl2-dev:i386
+```
+
+### Building
+
+```
+cmake -B build -S .
+cmake --build build
+```
+
+Note that the libraries built this way might be not compatible with Steam Half-Life. If you have such issue you can configure it to build statically with c++ and gcc libraries:
+```
+cmake .. -DCMAKE_C_FLAGS="-static-libstdc++ -static-libgcc"
+```
+
+## Linux x86. Build in your own chroot
+
+### Prerequisites
+
+Use the most suitable way for you to create an old distro 32-bit chroot. E.g. on Ubuntu/Debian you can use debootstrap.
+
+```
+sudo apt install debootstrap schroot
+sudo mkdir -p /var/choots
+sudo debootstrap --arch=i386 jessie /var/chroots/jessie-i386 # On Ubuntu type trusty instead of jessie
+sudo chroot /var/chroots/jessie-i386
+```
+
+```
+# inside chroot
+apt install cmake build-essential gcc-multilib g++-multilib libsdl2-dev
+exit
+```
+
+Create and adapt the following config in /etc/schroot/chroot.d/jessie.conf (you can choose a different name):
+
+```
+[jessie]
+type=directory
+description=Debian jessie i386
+directory=/var/chroots/jessie-i386/
+users=yourusername
+groups=adm
+root-groups=root
+preserve-environment=true
+personality=linux32
+```
+
+Insert your actual user name in place of `yourusername`.
+
+### Building
+
+Prepend any make or cmake call with `schroot -c jessie --`:
+```
+schroot --chroot jessie -- cmake -B build-in-chroot -S .
+schroot --chroot jessie -- cmake --build build-in-chroot
+```
+
+## Android
+
+TODO
+
+## Nintendo Switch
+
+### Prerequisites
+
+1. Set up [`dkp-pacman`](https://devkitpro.org/wiki/devkitPro_pacman).
+2. Install dependency packages:
+```
+sudo dkp-pacman -S switch-dev dkp-toolchain-vars switch-mesa switch-libdrm_nouveau switch-sdl2
+```
+3. Make sure the `DEVKITPRO` environment variable is set to the devkitPro SDK root:
+```
+export DEVKITPRO=/opt/devkitpro
+```
+4. Install libsolder:
+```
+source $DEVKITPRO/switchvars.sh
+git clone https://github.com/fgsfdsfgs/libsolder.git
+make -C libsolder install
+```
+
+### Building using CMake
+```
+mkdir build && cd build
+aarch64-none-elf-cmake -G"Unix Makefiles" -DCMAKE_PROJECT_CLIENT-MOD_INCLUDE="$DEVKITPRO/portlibs/switch/share/SolderShim.cmake" ..
+make -j
+```
+
+## PlayStation Vita
+
+### Prerequisites
+
+1. Set up [VitaSDK](https://vitasdk.org/).
+2. Install [vita-rtld](https://github.com/fgsfdsfgs/vita-rtld):
+   ```
+   git clone https://github.com/fgsfdsfgs/vita-rtld.git && cd vita-rtld
+   mkdir build && cd build
+   cmake -DCMAKE_BUILD_TYPE=Release ..
+   make -j2 install
+   ```
+
+### Building with CMake:
+```
+mkdir build && cd build
+cmake -G"Unix Makefiles" -DCMAKE_TOOLCHAIN_FILE="$VITASDK/share/vita.toolchain.cmake" -DCMAKE_PROJECT_CLIENT-MOD_INCLUDE="$VITASDK/share/vrtld_shim.cmake" ..
+make -j
+```
+
+## Other platforms
+
+Building on other architectures (e.g x86_64 or arm) and POSIX-compliant OSes (e.g. FreeBSD) is supported.
+
+### Prerequisites
+
+Install C and C++ compilers (like gcc or clang), cmake and make.
+
+### Building
+
+```
+cmake -B build -S .
+cmake --build build
+```
+
+## Build options
+
+Some useful build options that can be set during the cmake step.
+
+* **GOLDSOURCE_SUPPORT** - allows to turn off/on the support for GoldSource input. Set to **ON** by default on Windows and Linux, **OFF** on other platforms.
+* **USE_VGUI** - whether to use VGUI library. **OFF** by default. You need to init `vgui_support` submodule in order to build with VGUI.
+
+This list is incomplete. Look at `CMakeLists.txt` to see all available options.
+
+Prepend option names with `-D` when passing to cmake. Boolean options can take values **OFF** and **ON**. Example:
+
+```
+cmake .. -DUSE_VGUI=ON -DGOLDSOURCE_SUPPORT=ON -DCROWBAR_IDLE_ANIM=ON -DCROWBAR_FIX_RAPID_CROWBAR=ON
+```