ADDONS: make addons/ discoverable by git via .gitignore

When the contrib/ parts were moved to addons/, no rule was added in
.gitignore to make addons/ searchable for new files, let's fix it.
diff --git a/.gitignore b/.gitignore
index ae8b6a2..cd92620 100644
--- a/.gitignore
+++ b/.gitignore
@@ -19,6 +19,7 @@
 !/SUBVERS
 !/VERDATE
 !/VERSION
+!/addons
 !/admin
 !/dev
 !/doc