Add .editorconfig

Editorconfig is a lightweight specification that
helps maintaining consistent coding/formatting style
accross editors, especially those editors
that are not explicitly aware of Rust and rustfmt.

https://editorconfig.org/

.editorconfig

@@ -13,6 +13,8 @@ indent_style = space
 indent_size = 4
 
 [*.md]
+# double whitespace at end of line
+# denotes a line break in Markdown
 trim_trailing_whitespace = false
 
 [*.yml]