editorconfig: include markdown specifications

The markdown files in this project use spaces instead of tabs and
usually have an indent of 2.

.editorconfig

@@ -6,6 +6,10 @@ insert_final_newline = true
 trim_trailing_whitespace = true
 charset = utf-8
 
+[*.md]
+indent_style = space
+indent_size = 2
+
 [*.py]
 indent_style = space
 indent_size = 4