add pull bot configuration

Author: aminya

.github/pull.yml

@@ -0,0 +1,7 @@
+version: "1"
+rules:                      # Array of rules
+  - base: master            # Required. Target branch
+    upstream: atom:master   # Required. Must be in the same fork network.
+    mergeMethod: rebase     # Optional, one of [none, merge, squash, rebase, hardreset], Default: none.
+    mergeUnstable: false    # Optional, merge pull request even when the mergeable_state is not clean. Default: false
+label: ":arrow_heading_down: pull upstream"  # Optional