add .editorconfig file

.editorconfig

@@ -0,0 +1,9 @@
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true

test/layout_test.rb

@@ -36,6 +36,7 @@ describe "Repo layout" do
                     }
 
   TOPLEVEL_FILES = %w{
+                      .editorconfig
                       .gitignore
                       .rspec
                       .rubocop.yml