chore: add dependabot.yml file (#653)

This file is currently restricting to just Lotus, and is running daily 
to help catch any unexpected releases for API compat checks. 
If we decide to add more modules later, we could switch this
to run on Wednesday's UTC mornings, to catch the Tuesday RC 
releases from lotus, and avoid daily dependabot spam.

.github/dependabot.yml

@@ -0,0 +1,15 @@
+# This file is currently restricting to just Lotus, and is
+# running daily to help catch any unexpected releases for API compat
+# checks. If we decide to add more modules later, we could switch this
+# to run on Wednesday's UTC mornings, to catch the Tuesday RC releases from
+# lotus, and avoid daily dependabot spam.
+
+version: 2
+updates:
+  - package-ecosystem: "gomod"
+    directory: "/"
+    schedule:
+      interval: "daily"
+    allow:
+      - dependency-name: "github.com/filecoin-project/lotus"
+        dependency-type: "direct"