.editorconfig: Use spaces for .md files

While editing `readme.md` for chalkGH-17, I noticed that VS Code was
formatting lines with tabs for indentation and that it looked bad in the
diffs, because it wasn't matching the existing lines.

Ths updates `.editconfig` so that it uses spaces for indentation in
Markdown files, which seems to match what has been used.

.editorconfig

@@ -7,6 +7,6 @@ charset = utf-8
 trim_trailing_whitespace = true
 insert_final_newline = true
 
-[*.yml]
+[*.{md,yml}]
 indent_style = space
 indent_size = 2