add dependabot to create PR to update the submodules to latest (#467)

relates to riscv-admin/docs-sig#63

Signed-off-by: Kevin Broch <kbroch@rivosinc.com>

.github/dependabot.yml

@@ -0,0 +1,8 @@
+---
+# https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem
+version: 2
+updates:
+  - package-ecosystem: gitsubmodule
+    directory: /
+    schedule:
+      interval: daily