Add David as approver (#1255)

David meets all criteria for an approver:

- [x] Reviewer for at least 3 months
- [x] Primary reviewer for at least 10 substantial PRs to the codebase, e.g.
  * #1246
  * #1194
  * #738
  * #832
  * #1016
  * #877
  * #667
  * #697
  * #1212
  * #835
- [x] Reviewed at least 30 PRs to the codebase ([38 assigned PRs](https://github.com/knative/client/pulls?q=type%3Apr+assignee%3Adsimansk+))
- [x] Nominated by a WG lead (rhuss)

Congrats David ! Thanks a lot for your awesome work & contributions.

OWNERS

@@ -1,7 +1,7 @@
 approvers:
 - maximilien
+- dsimansk
 - client-wg-leads
 - toc
 reviewers:
 - itsmurugappan
-- dsimansk