docs: add extension policy

EXTENSION_POLICY.md

@@ -0,0 +1,58 @@
+# Envoy Extension Policy
+
+## Quality requirements
+
+All extensions contained in the main Envoy repository will be held to the same quality bar as the
+core Envoy code. This includes coding style, code reviews, test coverage, etc. In the future we
+may consider creating a sandbox repository for extensions that are not compiled/tested by default
+and held to a lower quality standard, but that is out of scope currently.
+
+## Adding new extensions
+
+The following procedure will be used when proposing new extensions for inclusion in the repository:
+  1. A GitHub issue should be opened describing the proposed extension as with any major feature
+  proposal.
+  2. All extensions must be sponsored by an existing maintainer. Sponsorship means that the
+  maintainer will shepherd the extension through design/code reviews. Maintainers can self-sponsor
+  extensions if they are going to write them, shepherd them, and maintain them.
+
+     Sponsorship serves two purposes:
+     * It ensures that the extension will ultimately meet the Envoy quality bar.
+     * It makes sure that incentives are aligned and that extensions are not added to the repo without
+     sufficient thought put into future maintenance.
+
+     *If sponsorship cannot be found from an existing maintainer, an organization can consider
+     [doing the work to become a maintainer](./GOVERNANCE.md#process-for-becoming-a-maintainer) in
+     order to be able to self-sponsor extensions.*
+
+  3. Each extension must have two reviewers proposed for reviewing PRs to the extension. Neither of
+  the reviewers must be a senior maintainer. Existing maintainers (including the sponsor) and other
+  contributors can count towards this number. The initial reviewers will be codified in the
+  [CODEOWNERS](./CODEOWNERS) file for long term maintenance. These reviewers can be swapped out as
+  needed.
+  4. Any extension added via this process becomes a full part of the repository. This means that any
+  API breaking changes in the core code will be automatically fixed as part of the normal PR process
+  by other contributors.
+
+## Removing existing extensions
+
+As stated in the previous section, once an extension becomes part of the repository it will be
+maintained by the collective set of Envoy contributors as needed.
+
+However, if an extension has known issues that are not being rectified by the original sponsor and
+reviewers or new contributors that are willing to step into the role of extension owner, a
+[vote of the maintainers](./GOVERNANCE.md#conflict-resolution-and-voting) can be called to remove the
+extension from the repository.
+
+## Extension pull request reviews
+
+Extension PRs must not modify core Envoy code. In the event that an extension requires changes to core
+Envoy code, those changes should be submitted as a separate PR and will undergo the normal code review
+process, as documented in the [contributor's guide](./CONTRIBUTING.md).
+
+Extension PRs must be approved by at least one sponsoring maintainer and an extension reviewer. These
+may be a single individual, but it is always preferred to have multiple reviewers when feasible.
+
+In the event that the Extension PR author is a sponsoring maintainer and no other sponsoring maintainer
+is available, another maintainer may be enlisted to perform a minimal review for style and common C++
+anti-patterns. The Extension PR must still be approved by a non-maintainer reviewer.

GOVERNANCE.md

@@ -116,8 +116,7 @@ the maintainers per the voting process below.
 # Extension addition policy
 
 Adding new [extensions](REPO_LAYOUT.md#sourceextensions-layout) has a dedicated policy. Please
-see [this](https://docs.google.com/document/d/1eDQQSxqx2khTXfa2vVm4vqkyRwXYkPzZCcbjxJ2_AvA) document
-for more information.
+see [this](./EXTENSION_POLICY.md) document for more information.
 
 # Conflict resolution and voting