add dependabot config (#279)

.github/dependabot.yml

@@ -0,0 +1,22 @@
+version: 2
+
+updates:
+  - package-ecosystem: github-actions
+    directory: '/'
+    schedule:
+      interval: daily
+  - package-ecosystem: cargo
+    directory: '/'
+    # Handle updates for crates from https://github.com/paritytech/polkadot-sdk manually.
+    ignore:
+      - dependency-name: 'sc-*'
+      - dependency-name: 'sp-*'
+      - dependency-name: 'frame-*'
+      - dependency-name: 'pallet-*'
+      - dependency-name: 'substrate-*'
+      - dependency-name: 'polkadot-*'
+      - dependency-name: 'cumulus-*'
+      - dependency-name: 'assets-*'
+      - dependency-name: 'xcm-*'
+    schedule:
+      interval: 'daily'