Create guide/overview for the SIG Security External Audit subproject (#5870)

* create a README for the external audit subproject

* add additional label

* remove adding label external-audit

* applied suggestions from aasmall

* applied 120 column length

sig-security/sig-security-external-audit/README.md

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+# SIG Security External Audit Subproject
+
+## Overview
+
+The SIG Security External Audit subproject (subproject, henceforth) is responsible for coordinating regular, 
+comprehensive, third-party security audits.
+The subproject publishes the deliverables of the audit after abiding to the 
+[Security Release Process](https://github.com/kubernetes/security/blob/master/security-release-process.md) and 
+[embargo policy](https://github.com/kubernetes/security/blob/master/private-distributors-list.md#embargo-policy).
+
+  - [Request for Proposal (RFP)](#rfp)
+    - [Security Audit Scope](#security-audit-scope)
+    - [Vendor and Community Questions](#vendor-and-community-questions)
+  - [Review of Proposals](#review-of-proposals)
+  - [Vendor Selection](#vendor-selection)
+  - [Deliverables](#deliverables)
+
+## RFP
+
+The subproject produces a RFP for a third-party, comprehensive security audit. The subproject publishes the RFP in the 
+`sig-security` folder in the `kubernetes/community` repository. The subproject defines the scope, schedule, 
+methodology, selection criteria, and deliverables in the RFP.
+
+Previous RFPs:
+  - [2019](https://github.com/kubernetes/community/blob/master/sig-security/security-audit-2019/RFP.md)
+  - [2021](https://github.com/kubernetes/community/blob/master/sig-security/security-audit-2021/RFP.md)
+
+As efforts begin for the year's security audit, create a tracking issue for the security audit in 
+`kubernetes/community` with the `/sig security` label.
+
+### Security Audit Scope
+
+The scope of an audit is the most recent release at commencement of audit of the core 
+[Kubernetes project](https://github.com/kubernetes/kubernetes) and certain other code maintained by 
+[Kubernetes SIGs](https://github.com/kubernetes-sigs/).
+
+Core Kubernetes components remain as focus areas of regular audits. Additional focus areas are finalized by the 
+subproject.
+
+### Vendor and Community Questions
+
+Potential vendors and the community can submit questions regarding the RFP through a Google form. The Google form is 
+linked in the RFP. 
+[Example from the 2021 audit](https://docs.google.com/forms/d/e/1FAIpQLScjApMDAJ5o5pIBFKpJ3mUhdY9w5s9VYd_TffcMSvYH_O7-og/viewform).
+
+The subproject answers questions publicly on the RFP with pull requests to update the RFP. 
+[Example from the 2021 audit](https://github.com/kubernetes/community/pull/5813).
+
+The question period is typically open between the RFP's opening date and closing date.
+
+## Review of Proposals
+
+Proposals are reviewed by the subproject proposal reviewers after the RFP closing date. An understanding of security audits is required to be a proposal reviewer.
+
+All proposal reviewers must agree to abide by the 
+**[Security Release Process](https://github.com/kubernetes/security/blob/master/security-release-process.md)**, 
+**[embargo policy](https://github.com/kubernetes/security/blob/master/private-distributors-list.md#embargo-policy)**, 
+and have no [conflict of interest](#conflict-of-interest) the tracking issue. 
+This is done by placing a comment on the issue associated with the security audit. 
+e.g. `I agree to abide by the guidelines set forth in the Security Release Process, specifically the embargo on CVE 
+communications and have no conflict of interest`
+
+Proposal reviewers are members of a private Google group and private Slack channel to exchange sensitive, confidential information and to share artifacts.
+
+### Conflict of Interest
+
+There is a possibility of a conflict of interest between a proposal reviewer and a vendor. Proposal reviewers should not have a conflict of interest. Examples of conflict of interest:
+  - Proposal reviewer is employed by a vendor who submitted a proposal
+  - Proposal reviewer has financial interest directly tied to the audit
+
+Should a conflict arise during the proposal review, reviewers should notify the subproject owner and SIG Security chairs when they become aware of the conflict.
+
+> The _Conflict of Interest_ section is inspired by the 
+[CNCF Security TAG security reviewer process](https://github.com/cncf/tag-security/blob/main/assessments/guide/security-reviewer.md#conflict-of-interest).
+
+## Vendor Selection
+
+On the vendor selection date, the subproject will publish a the selected vendor in the 'sig-security' folder in the `kubernetes/community` repository. 
+[Example from the 2019 audit](https://github.com/kubernetes/community/blob/master/sig-security/security-audit-2019/RFP_Decision.md).
+
+## Deliverables
+
+The deliverables of the audit are defined in the RFP e.g. findings report, threat model, white paper, audited reference architecture spec (with yaml manifests) and published in the 'sig-security' folder in the `kubernetes/community` repository. 
+[Example from the 2019 audit](https://github.com/kubernetes/community/tree/master/sig-security/security-audit-2019/findings).
+
+**All information gathered and deliverables created as a part of the audit must not be shared outside the vendor or the subproject without the explicit consent of the subproject and SIG Security chairs.**