Add ZhiminXiang to approvers.

From ROLES.md

1/ Reviewer of the codebase for at least 3 months.

knative#1687 is 1 year old.

2/ Primary reviewer for at least 10 substantial PRs to the codebase.

7 XLs: https://github.com/knative/serving/pulls?q=is%3Apr+assignee%3AZhiminXiang+is%3Aclosed+reviewed-by%3AZhiminXiang+label%3Asize%2FXXL
3 Ls: https://github.com/knative/serving/pulls?q=is%3Apr+assignee%3AZhiminXiang+is%3Aclosed+reviewed-by%3AZhiminXiang+label%3Asize%2FXL+
9 Ms:
https://github.com/knative/serving/pulls?q=is%3Apr+assignee%3AZhiminXiang+is%3Aclosed+reviewed-by%3AZhiminXiang+label%3Asize%2FM

3/ 33 Reviewed PRs
https://github.com/knative/serving/pulls?q=is%3Apr+assignee%3AZhiminXiang+is%3Aclosed+reviewed-by%3AZhiminXiang+

4/ 81 Merged PRs
https://github.com/knative/serving/pulls?q=is%3Apr+is%3Aclosed+author%3AZhiminXiang

4/ Nominated by nghia@ with no objections from other leads

OWNERS_ALIASES

@@ -58,6 +58,7 @@ aliases:
   - mdemirhan
   - tcnghia
   - vagababov
+  - ZhiminXiang
   networking-reviewers:
   - JRBANCEL
   - ZhiminXiang