Add extension creator to the credits page

Given that the credits page now includes many different contributors
(from translators to a list of open source projects used in the
extension), I thought it would be a good idea to add myself too.

Change-Id: Ie098d018e108af7ad2e48906ca75bcb9ec584198
diff --git a/src/_locales/en/messages.json b/src/_locales/en/messages.json
index 1f950f3..05c6fad 100644
--- a/src/_locales/en/messages.json
+++ b/src/_locales/en/messages.json
@@ -77,6 +77,10 @@
 		"message": "Credits",
 		"description": "Message for the link to the credits dialog, and also the title for the credits dialog, which shows a list of open source projects used inside the extension and a list of translators/contributors."
+  "options_credits_createdby": {
+    "message": "Extension created by: <a href=\"\" target=\"_blank\">@avm99963</a> (Adrià Vilanova Martínez)",
+    "description": "Text shown in the credits dialog, which shows that the extension was created by Adrià Vilanova Martínez (and their username is avm99963)."
+  },
 	"options_credits_homepage": {
 		"message": "homepage",
 		"description": "Text shown for links to go to each homepage of open source projects in the credits. NOTE: put in in lowercase letters"