add reference info

README.md

@@ -24,6 +24,25 @@ Future enhancements will include expansions to printf/sprintf with a custom form
 
 While it handles both additive and subtractive Roman numerals, it doesn't check that they're properly formatted. For instance 'IC' should be a compile-time error but instead it'll generate 101 as if nothing of consequence happened.
 
+REFERENCE
+=========
+
+According to [Wikipedia](https://en.wikipedia.org/wiki/Roman_numerals), the standard form is:
+
+| # | Thousands | Hundreds | Tens | Units |
+|---|-----------|----------|------|-------|
+| 1 | M         | C        | X    | I     |
+| 2 | MM        | CC       | XX   | II    |
+| 3 | MMM       | CCC      | XXX  | III   |
+| 4 |           | CD       | XL   | IV    |
+| 5 |           | D        | L    | V     |
+| 6 |           | DC       | LX   | VI    |
+| 7 |           | DCC      | LXX  | VII   |
+| 8 |           | DCCC     | LXXX | VIII  |
+| 9 |           | CM       | XC   | IX    |
+
+The numerals for 4 (IV) and 9 (IX) are written using subtractive notation,where the smaller symbol (I) is subtracted from the larger one (V, or X), thus avoiding the clumsier IIII and VIIII. Subtractive notation is also used for 40 (XL), 90 (XC), 400 (CD) and 900 (CM). These are the only subtractive forms in standard use.
+
 AUTHOR
 ======
 
@@ -34,7 +53,7 @@ Source can be located at: https://github.com/raku-community-modules/Slang-Roman
 COPYRIGHT AND LICENSE
 =====================
 
-Copyright 2016, 2018 Jeff Goff, 2020-2023 Raku Community
+Copyright 2016, 2018 Jeff Goff, 2020-2024 Raku Community
 
 This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.