Addition of include file description in STYLE_GUIDE

Author: jvdp1

STYLE_GUIDE.md

@@ -17,6 +17,7 @@ This style guide is a living document and proposed changes may be adopted after
 
 * Source files should contain at most one `program`, `module`, or `submodule`
 * The filename should match the program or module name and have the file extension `.f90` or `.F90` if preprocessing is required
+* All included files must use the `.inc` extension. These files should be located in the `include/` directory.
 * If the interface and implementation is split using submodules the implementation submodule file should have the same name as the
   interface (parent) module but end in `_implementation`
   E.g., `string_class.f90` and `string_class_implementation.f90`