Create pull.yml

安装[Pull](https://github.com/apps/pull)自动同步上游仓库，保持Fork后的仓库更新

.github/pull.yml

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