add link to rgaa standard

Author: straker

doc/API.md

@@ -94,7 +94,7 @@ The `experimental`, `ACT`, `TT`, and `section508` tags are only added to some ru
 | `TT*.*`           | Test ID in Trusted Tester                                                                                                     |
 | `EN-301-549`      | Rule required under [EN 301 549](https://www.etsi.org/deliver/etsi_en/301500_301599/301549/03.02.01_60/en_301549v030201p.pdf) |
 | `EN-9.*`          | Section in EN 301 549 listing the requirement                                                                                 |
-| `RGAAv4`          | Rule required under RGAA                                                                                                      |
+| `RGAAv4`          | Rule required under [RGAA](https://accessibilite.numerique.gouv.fr/methode/criteres-et-tests/)                                |
 | `RGAA-*.*.*`      | Section in RGAA listing the requirement                                                                                       |
 | `experimental`    | Cutting-edge rules, disabled by default                                                                                       |
 | `cat.*`           | Category mappings used by Deque (see below)                                                                                   |