Create dependabot.yml (#150)

Get automatic updates when some package we use (e.g. in GitHub actions) has a new version.

Author: kostis

.github/dependabot.yml

@@ -0,0 +1,16 @@
+# See the documentation for all configuration options:
+# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
+
+version: 2
+updates:
+  - package-ecosystem: "gitsubmodule"
+    directory: "/" # Location of package manifests
+    schedule:
+      interval: "weekly"
+
+  - package-ecosystem: "github-actions"
+    # Workflow files stored in the
+    # default location of `.github/workflows`
+    directory: "/"
+    schedule:
+      interval: "weekly"