Documentation: add a note about musl

Documentation/musl.md

@@ -0,0 +1,15 @@
+# Xash3D FWGS on `musl`
+
+Xash3D FWGS works on `musl` out of the box. However, the engine doesn't try to differentiate glibc and musl anymore. If you see error similar to:
+
+```
+Host_InitError: can't initialize cl_dlls/client.so: Error relocating valve/cl_dlls/client.so: __sprintf_chk: symbol not found
+```
+
+... or you know that the game you're running is linked against glibc, you can try using `libgcompat`, like this:
+
+```
+$ LD_PRELOAD=/lib/libgcompat.so.0 ./xash3d ...
+```
+
+It will automatically add the missing symbols that glibc binaries usually need. In the future we might automatically link engine against `libgcompat` for better compatibility with prebuilt or closed-source games, if there will be any use for this.