Update supported_languages.md

Author: jakewilliami

docs/src/supported_languages.md

@@ -6,3 +6,4 @@ Current languages to choose from are:
       - `:en_US` (aliases: `:en`)
       - `:en_UK` (aliases: `:en_GB`; equivalencies: `:en_NZ`, `:en_AU`)
     - Dictionaries supported include `:modern`, `:british`, and `:european`, and default to the former.
+  - Spanish (`:es`)