Remove line length from Markdown files (#36)

We do not honor the line length for Markdown files that we typically use
for code files in other repos. Some editors can show a marker at said
line length, but in this case it is distracting. Although it's not a
part of the EditorConfig spec, the `max_line_length` option is honored
by some editors such as Vim and can be used to remove the marker. This
commit sets that EditorConfig setting accordingly.

.editorconfig

@@ -0,0 +1,2 @@
+[*.md]
+max_line_length = unset