Introduce a CONTRIBUTING.md file with Lead requirements

CONTRIBUTING.md

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+# Kubeflow Manifests Contributor Guide
+
+Welcome to the Kubeflow project! We'd love to accept your patches and
+contributions to this project. Please read the
+[contributor's guide in our docs](https://www.kubeflow.org/docs/about/contributing/).
+
+The contributor's guide
+* shows you where to find the Contributor License Agreement (CLA) that you need
+  to sign,
+* helps you get started with your first contribution to Kubeflow,
+* and describes the pull request and review workflow in detail, including the
+  OWNERS files and automated workflow tool.
+
+## Leadership Roles
+
+The [kubeflow/manifests](https://github.com/kubeflow/manifests) repo is owned by
+the [Kubeflow Manifests Working Group](https://github.com/kubeflow/community/blob/master/wgs.yaml).
+
+Kubeflow's [governance model](https://github.com/kubeflow/community/blob/master/wgs/wg-governance.md)
+includes a plethora of different leadership roles.
+This section aims to provide a clear description of what these roles mean for
+this repo, as well as set expectations from people with these roles and requirements
+for people to be promoted in a role.
+
+A Working Group lead is considered someone that has either the role of
+**Subproject Owner**, **Tech Lead** or **Chair**. These roles were defined by trying
+to provide different responsibility levels for repo owners. For the Manifests WG
+we'd like to start by treating *approvers* in the root [OWNERS](https://github.com/kubeflow/manifests/blob/master/OWNERS),
+as Subproject Owners, Tech Leads and Chairs. This is done to ensure we have a
+simple enough model to start that people can understand and get used to. So for
+the Manifests WG we only have Manifests WG Leads, which are the root approvers.
+
+The following sections will aim to define the requirements for someone to become
+a reviewer and an approver in the root OWNERS file (Manifests WG Lead).
+
+### Manifests WG Lead Requirements
+
+The requirements for someone to be a Lead come from the processes and work required
+to be done in this repo. The main goal with having multiple Leads is to ensure
+that in case there's an absence of one of the Leads the rest will be able to ensure
+the established processes and the health of the repo will be preserved.
+
+With the above the main pillars of work and responsibilities that we've seen for
+this repo throughout the years are the following:
+1. Being involved with the release team, since the [release process](https://github.com/kubeflow/community/tree/master/releases) is tightly intertwined with the manifests repo
+2. Testing methodologies (GitHub Actions, E2E testing with AWS resources etc)
+3. Processes regarding the [contrib/addon](https://github.com/kubeflow/manifests/blob/master/contrib) components
+4. [Common manifests](https://github.com/kubeflow/manifests/tree/master/common)  maintained by Manifests WG (Istio, Knative, Cert Manager etc)
+5. Community and health of the project
+
+Root approvers, or Manifests WG Leads, are expected to have expertise and be able
+to drive all the above areas. Root reviewers on the other hand are expected to
+have knowledge in all the above and have as a goal to grow into the approvers
+role by helping with reviews throughout the project.
+
+#### Root Reviewer requirements
+
+* Primary reviewer for at least 2 PRs in `/common`
+* Primary reviewer for at least 1 PR in `/tests`
+* Primary reviewer for at least 1 PR in `/contrib`
+
+#### Root Approver requirements
+
+The goal of the requirements is to quantify the main pillars that we documented
+above. The high level reasoning is that approvers should have lead efforts and
+have expertise in the different processes and artefacts maintained in this repo
+as well as be invested in the community of the WG.
+
+* Need to be a root reviewer
+* Have been a WG liaison for a KF release
+* Has at least 3 substantial PRs merged for `/common`
+    * I.e. updating the versions of Istio, Dex or Knative
+    * I.e. update manifests to work with newer versions of kustomize
+* Has at least 2 substantial PRs merged for `/testing`
+* Has at least 1 substantial PR merged for `/proposals`
+* Has been an active participant in issues and PRs for at least 3 months
+* Has attended at least 75% of Manifests WG meetings