librashader build on Linux (#1465)

Added a section for building librashader on Linux

Didn't want this to get too long or distro specific so these should be
the absolute minimal instructions for the platform.

README.md

@@ -40,6 +40,19 @@ You should then be able to select SDL for input in the Settings > Drivers menu.
 ##### Building with clang
 
 clang++ is now the preferred compiler for ares. If clang is detected on Windows/macOS/BSD, it will be selected by default. On Linux and other platforms, g++ remains the default if present. To build with clang, it is necessary to install both the `clang` and `lld` packages. If you would like to manually specify a compiler, you can use the following option: `compiler=[g++|clang++]`  
+
+##### Librashader Support
+If you do not want to include librashader support, you can pass the following option to the `make` command to skip these requirements using: `librashader=false`
+
+Librashader requires rust in order to build. You can install it with the following command:
+``` 
+curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
+source ~/.cargo/env
+rustup toolchain install nightly
+```
+
+In order to build librashader, change into the `ares/thirdparty/librashader` directory and run the script `build-librashader.sh`
+Note that once the build completes, it will instruct you to run three copy commands to install the library on your system. These paths may be different depending on Linux distribution. 
   
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@@ -123,7 +136,7 @@ a26 fc sfc n64 sg ms md ps1 pce ng msx cv myvision gb gba ws ngp spec
 Build Output
 ------------
 
-There is a single binary produced at the end of compilation which can be found in `desktop-ui/out`. On OS's besides Linux, the `Database` & `Shader` directories are copied over here as well. On Linux, running `make install` after compilation will copy these directories and binary into suitable locations (see desktop-ui/GNUmakefile for details). Alternatively, these directories can be copied from `ares/Shaders/*` and `mia/Database/*`.
+There is a single binary produced at the end of compilation which can be found in `desktop-ui/out`. On OS's besides Linux, the `Database` & `Shader` directories are copied over here as well. On Linux, running `make install` after compilation will copy these directories and binary into suitable locations (see desktop-ui/GNUmakefile for details). Alternatively, these directories can be copied from `thirdparty/slang-shaders/*` into a `Shader` directory, and by copying `mia/Database/*`
 
 
 Command-line options