Add build instructions

BUILDING.md

@@ -0,0 +1,18 @@
+# On Windows
+`git clone --recurse https://github.com/IntriguingTiles/HLFixes`
+
+Open `HLFixes.sln` and build it. It should build with Visual Studio 2017 and up. I use VS2022 with MSVC v141_xp.
+
+# On Linux
+`git clone --recurse https://github.com/IntriguingTiles/HLFixes`  
+`cd Linux && mkdir build && cd build`
+
+If you're going to make a build that is intented to run on machines other than your own, you should build within the [Steam Runtime SDK](https://gitlab.steamos.cloud/steamrt/scout/sdk/-/blob/steamrt/scout/README.md).
+
+## With Steam Runtime (schroot)
+`schroot --chroot steamrt_scout_i386 -- cmake -DCMAKE_CXX_COMPILER=g++-9 -DCMAKE_C_COMPILER=gcc-9 ..`  
+`schroot --chroot steamrt_scout_i386 -- make`
+
+## Without Steam Runtime
+`cmake ..`  
+`make`