Update cadence for dependabot updates (#2402)

Summary:
Updates to `examples` aren't really an issue, since this is code the user runs in it's computer only when checking out draft.js. This PR makes dependabot post PRs for it monthly.
Pull Request resolved: #2402

Reviewed By: claudiopro

Differential Revision: D21443592

Pulled By: mrkev

fbshipit-source-id: 3865bda2ee612551bbe31010bd1123e579b936ff

.dependabot/config.yml

@@ -0,0 +1,8 @@
+version: 1
+update_configs:
+  - package_manager: "javascript"
+    directory: "/examples/draft-0-10-0/"
+    update_schedule: "monthly"
+  - package_manager: "javascript"
+    directory: "/"
+    update_schedule: "weekly"