Add .editorconfig file, configured as per contribution guidelines

See https://editorconfig.org/

For emacs

    (package-install 'editorconfig)
    (editorconfig-mode 1)

.editorconfig

@@ -0,0 +1,14 @@
+; This file is for unifying the coding style for different editors and IDEs.
+; More information at http://EditorConfig.org
+
+root = true
+
+[*]
+end_of_line = LF
+
+[*]
+indent_style = space
+indent_size = 2
+trim_trailing_whitespace = true
+insert_final_newline = true
+max_line_length = 80