chore: add CODEOWNERS (argoproj#641)

* chore: add CODEOWNERS and EMERITUS.md

Setting up CODEOWNERS file so people are automatically notified about new PRs. Can
eventually setup ruleset that requires at least one review before merging.

I based currently list in CODEOWNERS on people who have recently merged PRs. I'm wondering
if reviewers and approvers are a team/group instead of list of folks.

Setup EMERITUS.md that contains list from OWNERS.  Feedback on this PR will update
this PR.

Signed-off-by: jmeridth <[email protected]>

* chore: match this repo's CODEOWNERS to argoproj/argo-cd CODEOWNERS

Signed-off-by: jmeridth <[email protected]>

---------

Signed-off-by: jmeridth <[email protected]>

Author: jmeridth

.github/CODEOWNERS

@@ -0,0 +1,10 @@
+# All
+** @argoproj/argocd-approvers
+
+# Docs
+/docs/**     @argoproj/argocd-approvers @argoproj/argocd-approvers-docs
+/README.md   @argoproj/argocd-approvers @argoproj/argocd-approvers-docs
+
+# CI
+/.codecov.yml             @argoproj/argocd-approvers @argoproj/argocd-approvers-ci
+/.github/**               @argoproj/argocd-approvers @argoproj/argocd-approvers-ci