configure backport tool (#143)

Author: jwilliams-mongo

.backportrc.json

@@ -0,0 +1,14 @@
+{
+    "repoOwner": "10gen",
+    "repoName": "docs-atlas-operator",
+  
+    "targetBranchChoices": ["v2.4", "v2.3", "v2.2", "v1.9"],
+  
+    "editor": "code",
+  
+    "maxNumber": 15,
+  
+    "backportBranchName": "{{sourcePullRequest.number}}-{{targetBranch}}-backport",
+  
+    "prTitle": "{{sourcePullRequest.title}}-{{targetBranch}}-backport ({{sourcePullRequest.number}})"
+  }