Configure Renovate (#539)

* Add renovate.json

* Add golang options

- gomodUpdateImportPaths to support lego/v4 imports
- gomodTidy as recommended by https://github.com/ampproject/amppackager/wiki/Contributing#new-dependencies

* Change base branch from default (releases) to main

Co-authored-by: renovate[bot] <renovate[bot]@users.noreply.github.com>
Co-authored-by: Devin Mullins <[email protected]>

Author: renovate[bot]

renovate.json

@@ -0,0 +1,10 @@
+{
+  "baseBranches": ["main"],
+  "extends": [
+    "config:base"
+  ],
+  "postUpdateOptions": [
+    "gomodUpdateImportPaths",
+    "gomodTidy"
+  ]
+}