Added auto-reviewer config file (#13962)

.github/auto_assign.yml

+# This is the config file for `auto-assign` bot.
+# https://github.com/kentaro-m/auto-assign/
+
+# Set to true to add reviewers to pull requests
+addReviewers: true
+
+# Set to true to add assignees to pull requests
+addAssignees: false
+
+# A list of reviewers to be added to pull requests (GitHub user name)
+# Note: Add new engine contributors here when joining the team.
+reviewers:
+  - gaaclarke
+  - liyuqian
+  - gw280
+  - chinmaygarde
+  - GaryQian
+  - jason-simmons
+  - iskakaushik
+  - franciscojma86
+  - cbracken
+  - flar
+  - stuartmorgan
+
+# A number of reviewers added to the pull request
+# Set 0 to add all the reviewers (default: 0)
+numberOfReviewers: 1
+
+# A list of assignees, overrides reviewers if set
+# assignees:
+#   - assigneeA
+
+# A number of assignees to add to the pull request
+# Set to 0 to add all of the assignees.
+# Uses numberOfReviewers if unset.
+# numberOfAssignees: 2
+
+# A list of keywords to be skipped the process that add reviewers if pull requests include it
+# skipKeywords:
+#   - wip