editorconfig: Use 2-column spaces in YAML

.editorconfig

@@ -1,6 +1,10 @@
-
 root = true
 
 [*]
 indent_style = tab
 end_of_line = lf
+
+[*.y{,a}ml]
+indent_size = 2
+indent_style = space
+tab_width = 2