Update Contributing.md doc (#194)

Author: reyang

CONTRIBUTING.md

@@ -8,6 +8,11 @@ See the [public meeting notes](https://docs.google.com/document/d/1CIMGoIOZ-c3-i
 for a summary description of past meetings. To request edit access, join the
 meeting or get in touch on [Gitter](https://gitter.im/open-telemetry/opentelemetry-python).
 
+See to the [community membership document](https://github.com/open-telemetry/community/blob/master/community-membership.md)
+on how to become a [**Member**](https://github.com/open-telemetry/community/blob/master/community-membership.md#member),
+[**Approver**](https://github.com/open-telemetry/community/blob/master/community-membership.md#approver)
+and [**Maintainer**](https://github.com/open-telemetry/community/blob/master/community-membership.md#maintainer).
+
 ## Development
 
 This project uses [`tox`](https://tox.readthedocs.io) to automate some aspects
@@ -74,15 +79,16 @@ Open a pull request against the main `opentelemetry-python` repo.
 ### How to Get PRs Merged
 
 A PR is considered to be **ready to merge** when:
-* It has received two approvals from Collaborators/Maintainers (at different
-  companies).
+* It has received two approvals from [Approvers](https://github.com/open-telemetry/community/blob/master/community-membership.md#approver)
+  / [Maintainers](https://github.com/open-telemetry/community/blob/master/community-membership.md#maintainer)
+  (at different companies).
 * Major feedbacks are resolved.
 * It has been open for review for at least one working day. This gives people
   reasonable time to review.
 * Trivial change (typo, cosmetic, doc, etc.) doesn't have to wait for one day.
 * Urgent fix can take exception as long as it has been actively communicated.
 
-Any Collaborator/Maintainer can merge the PR once it is **ready to merge**.
+Any Approver / Maintainer can merge the PR once it is **ready to merge**.
 
 ## Design Choices
 
@@ -111,25 +117,3 @@ For a deeper discussion, see: https://github.com/open-telemetry/opentelemetry-sp
   as specified with the [napolean
   extension](http://www.sphinx-doc.org/en/master/usage/extensions/napoleon.html#google-vs-numpy)
   extension in [Sphinx](http://www.sphinx-doc.org/en/master/index.html).
-
-## Become a Collaborator
-
-Collaborators have write access to the repo.
-
-To become a Collaborator:
-* Become an active Contributor by working on PRs.
-* Actively participate in the community meeting, design discussion, PR review
-   and issue discussion.
-* Contact the Maintainers, express the willingness and commitment.
-* Acknowledged and approved by two Maintainers (at different companies).
-
-## Become a Maintainer
-
-Maintainers have admin access to the repo.
-
-To become a Maintainer:
-* Become a [member of OpenTelemetry organization](https://github.com/orgs/open-telemetry/people).
-* Become a Collaborator.
-* Demonstrate the ability and commitment.
-* Contact the Maintainers, express the willingness and commitment.
-* Acknowledged and approved by all the current Maintainers.