add julia package-ecosystem to dependabot

Author: ablaom

.github/dependabot.yml

@@ -5,3 +5,9 @@ updates:
     directory: "/" # Location of package manifests
     schedule:
       interval: "monthly"
+  - package-ecosystem: "julia"
+    directories: # Location of Julia projects
+      - "/"
+      - "/docs"
+    schedule:
+      interval: "weekly"