Create dependabot.yml (#1615)

Enable Dependabot updates to Cargo and Github Actions dependencies.

As discussed in #1610.

.github/dependabot.yml

@@ -0,0 +1,21 @@
+# Please see the documentation for all configuration options:
+# https://docs.github.com/github/administering-a-repository/configuration-options-for-dependency-updates
+
+version: 2
+updates:
+  - package-ecosystem: cargo
+    directory: /
+    schedule:
+      interval: weekly
+    reviewers:
+      - djmitche
+      - mgeisler
+      - qwandor
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly
+    reviewers:
+      - djmitche
+      - mgeisler
+      - qwandor