Add .editorconfig file.

Vim doesn't honour the modelines by default because they're too far from
the start/end of the file [the default value of the 'modelines' (sic)
option is too small].

.editorconfig

@@ -0,0 +1,9 @@
+# Top-most editorconfig file
+
+root = true
+
+[*]
+end_of_line = lf
+tab_width   = 2
+indent_size = 2
+indent_style = space