add backport json (#244)

Author: shuangela

.backportrc.json

@@ -0,0 +1,19 @@
+{
+    // Required
+    "repoOwner": "mongodb",
+    "repoName": "docs-spark-connector",
+  
+    // the branches available to backport to
+    "targetBranchChoices": [
+      "master",
+      "v10.4",
+      "v10.3",
+      "v10.2",
+      "v10.1",
+      "v10.0"
+    ],
+  
+    // Optional: automatically merge backport PR
+    "autoMerge": true,
+    "autoMergeMethod": "squash"
+  }