Add CODEOWNERS file and clarify ownership in repo readme (#140)

* clarify repo ownership by integrations team

* add template level code ownership in CODEOWNERS file

Author: tommyers-elastic

.github/CODEOWNERS

@@ -0,0 +1,4 @@
+* @elastic/obs-infraobs-integrations
+
+/assets/templates/ec2* @elastic/obs-ds-hosted-services
+/assets/templates/kubernetes* @elastic/obs-cloudnative-monitoring 

README.md

@@ -15,8 +15,7 @@ Want to explore use cases? See [usage](./docs/usage.md).
 
 # Maintainers
 
-[@aspacca](https://github.com/aspacca)
-[@endorama](https://github.com/endorama)
+[Observability Integrations Team](https://github.com/orgs/elastic/teams/obs-infraobs-integrations)
 
 # License