Add a simple EditorConfig coding style file

EditorConfig seems to be supported by more and more editors/IDEs, including
even the upcoming MSVS 2017, so it seems useful to provide its config file.

.editorconfig

@@ -0,0 +1,16 @@
+# This is the EditorConfig (http://editorconfig.org/) coding style file for
+# wxWidgets.
+
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+indent_style = space
+indent_size = 4
+trim_trailing_whitespace = true
+
+[makefile.*]
+indent_style = tab
+indent_size = 8