Docs: make editorconfig for CMake explicit (4-space indents)

.editorconfig

@@ -7,6 +7,16 @@ charset = utf-8
 end_of_line = lf
 trim_trailing_whitespace = true
 
+[CMakeLists.txt]
+indent_style = space
+indent_size = 4
+insert_final_newline = true
+
+[*.cmake]
+indent_style = space
+indent_size = 4
+insert_final_newline = true
+
 [*.{py,cpp,h}]
 indent_style = space
 indent_size = 4