Fix CMake custom script (#658)

* CMake - CMake now detects custom scripts in the Custom directory

* CMake - CMake now detects the custom script headers

* CMake - Added an example in the README on how to make a CMakeLists.txt for a custom script

src/scripts/ScriptLoader.h

@@ -7,6 +7,7 @@
 #ifndef SC_SCRIPTLOADER_H
 #define SC_SCRIPTLOADER_H
 
+void AddCustomScripts();
 void AddScripts();
 void AddSpellScripts();
 void AddCommandScripts();