Add Dependabot (#63)

Dependabot will automatically file PRs for updatable dependencies. The cargo section watches
`Cargo.toml` while the github-actions section watches the workflow files in `.github/workflows` for
possible updates to any of the action in use.

Author: LingMan

.github/dependabot.yml

@@ -0,0 +1,10 @@
+version: 2
+updates:
+  - package-ecosystem: "cargo"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"