Added instructions for installing on arch linux

Working-on-GNU-Linux.md

@@ -170,6 +170,13 @@ Continue with:
 cc main.c -lraylib -lGL -lm -lpthread -ldl -lrt -lX11
 ```
 
+### Arch Linux
+The official Arch Repository contains the `raylib` package. Install via:
+
+```
+pacman -S raylib
+```
+
 ## Common issues
 
 #### I get an error: `error while loading shared libraries: libraylib.so.351: cannot open shared object file: No such file or directory` after rebooting. Right after building the library it worked fine.