MAINT: adding dependabot (#215)

Author: bsipocz

.github/dependabot.yml

@@ -0,0 +1,14 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: ".github/workflows"
+    schedule:
+      interval: "monthly"
+    cooldown:
+      default-days: 14
+    groups:
+      actions:
+        patterns:
+          - "*"
+    labels:
+      - "github_actions"