chore: promote Robb Kidd to Approver (#100)

* chore: promote Robb Kidd to Approver

Follow-up to the conversation[1] in CNCF Slack #otel-ruby, Robb Kidd
would like to help more with reviews and contributions.

[1] https://cloud-native.slack.com/archives/C01NWKKMKMY/p1657664615160479

* docs: add Approvers section to Contributing

... now that there are Approvers for ruby-contrib

* docs: correct link to contrib maintainers team

CODEOWNERS

@@ -12,4 +12,4 @@
 #  https://help.github.com/en/articles/about-code-owners
 #
 
-* @fbogsany @mwear @robertlaurin @dazuma @ericmustin @arielvalentin @ahayworth @plantfansam
+* @fbogsany @mwear @robertlaurin @dazuma @ericmustin @arielvalentin @ahayworth @plantfansam @robbkidd

README.md

@@ -25,7 +25,13 @@ requests.
 The Ruby special interest group (SIG) meets regularly. See the OpenTelemetry
 [community page][ruby-sig] repo for information on this and other language SIGs.
 
-Maintainers ([@open-telemetry/ruby-maintainers](https://github.com/orgs/open-telemetry/teams/ruby-maintainers)):
+Approvers ([@open-telemetry/ruby-contrib-approvers](https://github.com/orgs/open-telemetry/teams/ruby-contrib-approvers)):
+
+- [Robb Kidd](https://github.com/robbkidd), Honeycomb
+
+*Find more about the approver role in [community repository](https://github.com/open-telemetry/community/blob/master/community-membership.md#approver).*
+
+Maintainers ([@open-telemetry/ruby-contrib-maintainers](https://github.com/orgs/open-telemetry/teams/ruby-contrib-maintainers)):
 
 - [Ariel Valentin](https://github.com/arielvalentin), GitHub
 - [Daniel Azuma](https://github.com/dazuma), Google