docs: add an explanatory note to commify.html

output/_includes/commify.html

@@ -1,3 +1,10 @@
+{%- comment -%}
+This is a ludicrously complicated way of thousands separators to large numbers.
+
+I couldn’t figure out a better way of doing it without requiring either Ruby or
+Javascript.
+{%- endcomment -%}
+
 {%- assign number_string = include.number | split: '' -%}
 {%- assign total_digits = number_string.size -%}
 {%- assign commify_number = '' -%}